I'm going out on a limb here, but just give me a shot.
I know I know, you specifically requested something other than this because of the discovery date, but for the sake of argument let's explore it a little more.
Other answers have provided a lot of creative and neat alternatives, however none of them would make a practical sword. The radium solution would let you have an actual, usable sword, that would glow very nicely, even in a range of colors. Your main problem with it just seems to be that it was, in our history, discovered a hair past the deadline. That is really the only weak link in the solution, but you could easily work around that without requiring suspension of disbelief.
Not all of ancient history is well-documented.
There's plenty we don't know about our own history, even during the time of written records. For crying out loud, we're still (somewhat) arguing over how the Egyptians moved some rocks around. It's really not out of the realm of possibility that radium was, even by accident, isolated much earlier than 1911, and the details of how and where were simply lost. Virtually every major religious text mention someone at some point wielding a flaming sword, or similar fantastical implement. We often dismiss those as just stories, but a lot of what appears magical in the past was likely inspired by actual events that were simply misunderstood.
You're going for effect, right?
If someone crafted a glowing sword after 1911, there would be zero mystery and therefore nobody would pay attention beyond going "Oh that's neat." If you want a powerful king to have a fantastic weapon, it's actually necessary that the methods of its creation be unknown to the public. Maybe the king himself doesn't even know the secret, it was just gifted to him by a legendary swordsmith or alchemist who may or may not fully understand how they made it.
Okay so now the bad news. Radium, of course, is radioactive and hazardous. It also can't be applied in a permanent fashion, but luckily the phosphorescent coating that is the actual glowing part can be re-applied as needed. The hazards of the radiation aren't trivial, but they also aren't totally detrimental. If your king is only carrying the sword into battle or for ceremonial purposes, and it is safely stored otherwise, he could say relatively safe from it. Also your swordsmith would be at risk, but if he learned from a predecessor or even from experience, he would probably know that something about the magical material was unsafe, and could limit his own exposure so that it doesn't hurt him too badly.