Hi Michael,
I had a very black tongue when undergoing radiotherapy. The Radio registrar recommended Nilstat drops, Fungilin lozenges. When Nilstat was unavaiable in this country for a few months I tried Daktarin oral gel which works just as well. My black tongue went away after a while, and I occasionally get thrush still (almost 2 years since I finished radiation).
Hope this helps.
Cheers!
Tizz