It still won't be permanent, but water-slide decals will probably work better than stickers, and about as well as the marker while being more legible. It might

It still won't be permanent, but water-slide decals will probably work better than stickers, and about as well as the marker while being more legible. It might also be possible to put a clear coat paint on it that'll help them last longer. The big advantage over stickers is that they aren't held on by glue, but instead by the plastic that they're made out of conforms to the surface and grips it instead. That said I'm not sure how to make white text for a black keyboard.