For a science-based question, I'll tackle it slightly differently and re-frame it. It doesn't make a ton of sense for there to be living organisms with acid for blood like the xenomorphs.
So, I'd take a different reading of your question: you want your aliens to be dangerous if causing open wounds because of what the (human) stabber will be exposed to. So, not necessarily blood and not necessarily alkaline content.
They have a substance in their body their that is toxic to humans
Now, I'd discuss a number of possibilities here. I'm going to pick the ones that seem the coolest to me and further explore them, but you can always go in a different direction from something I've mentioned but not expanded on.
What is the substance?
This could actually still be blood. But it might not be, depending on physiology. They could have, say, a subdermal layer of the substance that helps them somehow. It might be to keep them warm, it might be specifically to protect from injuries. Maybe it's even a defence mechanism against predators. It might also be a property of their blood, not necessarily a separate substance. You can tailor as appropriate.
Why is it dangerous?
The simplest answer is that maybe it's not even supposed to be but it's dangerous to humans. But we can delve a little deeper.
There are species of animals in the real world that have evolved defences like these. In their case, it's usually against being eaten. There are the poisonous animals like poison dart frogs. The rationale is that if they are eaten, the predator will suffer greatly or die. This doesn't help the individual animals, but over time there won't be many predators left that will prey on the poisonous animals. In addition, poisonous animals tend to be very brightly coloured as a "don't eat me" mechanism. This all means that over time, the poisonous species will be protected from predators.
Just because this is cool, I'll add another example - the skink is a lizard with green blood and most internals. It's apparently green because it contains a large concentration of a chemical called biliverdin which is toxic. From the article:
If humans have even a tiny amount of biliverdin or bilirubin in their blood, we say that they are jaundiced (their skin takes on a yellowish tone).
[...]
if excess bilirubin or biliverdin in the bloodstream goes untreated, it can be deadly.
“It’s surprising because at these concentrations of bile pigments in the blood, [the skinks] should be completely jaundiced, if not dead,” Austin said.
The researcher who studies skinks thinks that is an evolution to protect from parasites, rather than predators.
With all that said, it's not at all unusual to have a substance that is dangerous to others. Or maybe just to humans, but this would suggest it's rather random...unless like the xenomorph, the alien adapts to given species.
How did it evolve?
A defensive mechanism seems a very likely candidate. It needs to be work through skin contact not necessarily through being consumed. This makes it eligible for a "coating" of the substance on the skin so just touching might trigger poisoning the attacker but any open wounds might still spread the defensive toxin. Perhaps instead it's being shot out by glands on the body that gets triggered the creature is attacked (I assume automatic, rather than at will).
Still, if you insist on open wounds being that thing that spreads the toxic substance, this is doable and also has interesting implications - a very strong defensive mechanism that serves as an extreme deterrent to open wounds, suggests there is dangerous fauna (or maybe even flora?) in the aliens' natural environment that causes puncture wounds. So, species which use claws, horns, or even teeth, as well as other spiky or cutting parts to cause open wounds. Such methods of causing injuries will be countered very well by having the attacker suffer for spilling blood.
This can give you an idea of what other species native to the main alien's world there would be. Or maybe shape the ability based on what you've already figured the alien fauna is.
At any rat, since toxic substances are likely to be slower acting than directly burning enemies with heavily acidic or alkaline blood, then this also has interesting implication - the alien with this defensive mechanism: it can survive well even when injured, for the defensive mechanism expects the attacker to first cause a wound and some time later they'd suffer for it. The target of the attack has to survive for the time the poison takes effect.
- One option is that a wound not only delivers a toxin to the attacker but also causes the target to get an adrenal (or equivalent?) burst, so they can either escape more easily or fight and defend themselves better.
- Another option is that the alien has evolved to be very sturdy. Likely there are redundant organs and very thick external layer. In this case, the poison could be a subdermal layer that serves as extra protection. For example, it could be useful for shock absorption thus lessening the effects of any blows that don't pierce outer layer or don't pierce it enough. If an attack opens a "wound" (it's likely for the outer layer to be more defensive, so opening it is not going to equate to damage to the body) then the toxic substance leaks (or shoots) out as a retaliation. The alien can still fight the attacker relying on outliving them.
In conclusion
I picture a sturdy alien that is probably generally slow to run away from natural predators in its environment but with very good endurance, resistance to pain, and survivability. If it gets into a fight, it relies more on outlasting opponents than just beating them through sheer strength or speed. It has a subdermal layer of toxic substance that can help with shock absorption and if the epidermis is punctured, the shock absorbing substance coats the attacker seeping into their system and poisoning them. This serves as a way to shorten the fight - causing open wounds is bad, no matter how good your outer layer is - if an attacker can open it, then might be able to attack through that opening. So, the poison is a way to deal with this more directly and reduce the window the attacker has to cause serious damage. Eventually, they would be incapacitated by the effects of this defensive poison and the poisonous creature is expected to be sturdy enough to survive that long.
Overall, perhaps this could be loosely like a hippo or rhino - quite well defended and not a pushover. I'd expect them to be able to survive even a decent cut or a piercing wound. But they are also poisonous.
When facing humans, this works the same way - if you were to cut them with a knife or other similar weapons, that's not much different from what other animals will attack with. A good suit will protect the human from the toxic substance but they still have to deal with actually killing the alien creature which should take quite some effort. Attacks that don't cause open wounds are not that effective, so if shooting them, normal bullets might be at least partly mitigated if the outer layer of the creatures is hard enough to stop them, then the subdermal layer will additionally absorb some of the kinetic force. Using armour piercing ammo will likely work but they generally cause far less damage to tissue because they tend to go straight through. Also, shooting the alien full of holes will turn them into a sprinkler for poison, you better stand back and only examine the body once properly protected.
