Weaken essential parts instead of the whole thing
My source for all of this is that I work at a historical armoury and have actually assembled/disassembled quite a few pieces for restoration.
I know which damages are fixed easily and which ones take ages. I also know which ones render a weapon/armor useless and which ones don't.
It's pretty easy to discern accidental change from something a person did on purpose - in my case the deliberate changes were made by past restaurators but seeing as their goal is also to not have their work noticed I think the methods would be the same as a saboteurs - only the goal is the complete opposite.
I think it wouldn't be easy to sabotage the actual armor plates or blades. Even after centuries they are still basically unchanged and usable. Any sabotage that effectively weakens a plate would probably be visible or at least discernible as sabotage.
Likewise a good blacksmith would notice changed raw materials or sabotage during production.
Also all armor, weapons and especially guns were tested before being accepted by the army. This means that any major faults like a suit of armor breaking on the first hit would be noticed before they even leave the armourers workshop.
Therefor I would suggest to go for the things keeping it all together. After all what use is a suit of armor if you can't fix it to your body?
Sabotage: Armor, scabbards, guns
You could have them treat the leather straps with oil that has gone bad. This will turn the leather acidic which will cause it to go weak and break.
Upside: The chemistry of why the leather breaks wasn't yet known in the middle ages so it would probably be blamed on the blacksmith using poor materials.
Downside: It takes a few years.
Complexity of fixing it: You need to replace the leather straps - for armor that is complete disassembly and reassembly. Scabbards and guns: quick fix
Source: This actually happened accidentally after WWII when supplies were short and the armouries batch of petroleum had gone bad. This is the reason why there are so few original leather pieces left.
(btw: do NOT use petroleum for leather! Even if it hasn't gone bad it's not a good idea)
Sabotage: Armor, scabbards, polearms, shields
Every piece of a suit of armor is connected to another part or a strap by rivets.
If you don't know how handmade rivets in armor works: It's basically a nail that is stuck through a tiny hole in the plate and let's say a leather strap and then flattened with a hammer on the other side.
Your saboteurs could file off that flattened piece of the rivet. Not completely so it doesn't fall out immediately but if it's just slightly bigger than the hole in the leather strap and plate it will eventually slip through with movement which leads to the armor falling apart.
Upside: it would look exactly like shabby workmanship but wouldn't be spotted by a rough inspection
Complexity of fixing it: Depending on which rivet breaks you might have to disassemble the armour completely. Scabbards and Polearms: quick fix
Source: I've seen this happen when we had to replace the leather strap and the new rivet wasn't done properly.
Sabotage: Swords, sabres, daggers...
A sword isn't much use without a hilt. Mostly the blade has a smaller metal part protruding from the back. The crossguard is fixed to that piece and then leather/wood/wire is wrapped around it to form the actual handle.
If the leather/wire is loosened the handle will come apart leaving you with an acceptable blade you can't wield. If the handle is wood the saboteurs could saw off a tiny bit on one end so that it moves a bit between the crossguard and the pommel. It doesn't actually destroy anything but makes the sword awful to swing. If they saw off just a tiny bit it would look like the blacksmith fitted the handle poorly.
Complexity of fixing it: reasonably quick fix
Source: Quite a few handles have loosened by age and some of the wooden handles in the armoury have shrunk giving that loose effect.
Soldiers don't run around with drawn weapons all the time so sabotaging the scabbards is also worth a shot. You can do the same thing as I described above with the armor rivets.
You could also make slits or holes in the outer layer of leather. It normally prevents moisture from reaching the wooden part inside. If the slit is directly next to a metal band or the tip protector it might look like the blacksmith damaged the leather when assembling it.
Not so much for discrediting the blacksmith but still useful: You can also pour blood or salt water into the scabbard which will cause the sword to rust in its scabbard. (This is the reason why they have to be cleaned before being put back into the scabbard) Combined with a slit here and there and if you're lucky people assume the liquid got in through the slits which would be the fault of the blacksmith.
Upside: looks like the soldiers weren't careful with their weapons.
Downside: rusting takes a few days to weeks - depending on other circumstances.
Complexity of fixing it: if the blade is just rusty: reasonably quick fix, if it is rusted stuck in the scabbard: they would probably cut the scabbard off the blade and make a new one so reasonable to long fix (in the museum we have to preserve both)
Most pole weapons are mounted to the wooden staff by metal bands that run down the wood and are nailed to it.
These nails can be loosened quite easily. The can also be filed off a bit like rivets.
Complexity of fixing it: quick fix
Source: Shrinkage of wood over time has caused lots of our poleweapons to have the nails come loose and they have become wobbly.
I don't know if there are firearms in your universe but if there are your saboteurs might want to refrain from doing the obvious and corrupting the lock. That's the first place one would look for sabotage. Seeing as the mechanism of a flintlock is actually pretty simple any damage would be quickly spotted by a blacksmith as soon as they take the lock off the gun.
Every gun was inspected and tested after being finished so again I would instead opt for the accessories. After all a gun without powder is useless.
It's hard to pin this on the blacksmith though. One way of doing that would be to temper with some of the black powder. If the powder is more powerful it might explode the barrel of the gun. This also happened with poorly crafted barrels. If too many barrels explode the blame will be put on whoever made the powder though.
Upside: if the gun explodes you also take out a soldier at the same time
Complexity of fixing it: an exploded barrel can't be fixed
Source: lots of people have tried shooting antique firearms but they used modern powder. Modern powder is more powerful and explodes old guns.
Gun barrels are fixed to the wood by (most of the time) two splints and one two three screws. All of the screws are in the back next to the lock while the splints are placed along the length of the barrel.
Pull out those splints and all the strain is put on the screws. They are relatively small and sometimes go missing on their own. It could either be that the blacksmith forgot them in the first place or made them too thin so they fell out.
Complexity of fixing it: quick fix
Source: So many old weapons missing the splints.
Bolts and Screws
Sabotage: guns, some swords, some armor
Screw them in extremely tight so there is more pressure on the screw. This might snap off the head or damage the thread if there is some fault in the material already.
Damage the slot so it can't be used properly anymore. This usually happens by accident when fixing something but it could also be done on purpose.
Upside: soldiers don't usually touch the screws so any damage to them would only be noticed by a blacksmith who would then know that the one previously handling the object was an idiot who didn't use a fitting screwdriver - discredit the blacksmith among his peers!
Downside: mangled screws are only a problem once you want to fix the object because it has another problem
Complexity of fixing it: very annoying reasonable fix
Source: If the slot of a screw is mangled we actually have to fit a screwdriver to it so we can get the screw out
Sabotage: swords, guns, armor
Mix swords and scabbards so they don't fit properly anymore. Same with armor parts like the front and back of a suit of armor.
With guns it's even easier because soldiers had to make their own bullets. All of that equipment is handmade and fitted to the barrel - switch it up with another and they don't fit properly anymore.
Also if you take two guns and switch out their locks it might even render the gun completely useless. There is a small hole in the barrel right were the pan on the lock is. When firing this is were the spark from the powder on the pan goes through to ignite the powder in the barrel. Because no two guns are exactly the same the small hole might be off or even covered by the pan if the wrong lock is fixed to the barrel.
If confronted about it the blacksmith himself will probably realise what happened but it's something you can't really prove. There are no serial numbers to show which parts belong together and to a layman they all look the same.
Some of these will be harder to pull off than others and some have the added benefit of annoying the army blacksmith who has to fix it.
All of these equipment failures would probably happen anyway so with your sabotage you are just going to increase the frequency. Of course if suddenly everything goes wrong all at once people will get suspicious especially if the blacksmith has always delivered quality work before.
None of these are particularly big but all of them show a lack of attention or skill if they happen to fairly new weapons.
So even though the blades and plates this blacksmith makes are actually pretty good he will earn a reputation of not taking care of details and slacking with the unglamorous parts.