Black!
I used Dylon multi-purpose dye, roughly following
this thread, although not in a very scientific way.
I stained a few fingernails in the process, but the keys turned out perfect! Even better than I expected. Every key is completely black, and the indices are just slightly visible since they aren't flush with the surface to begin with.
(Ignore the unfinished fox key, there's a red esc key in the mail that will soon take its place.
)