I actually stumbled across this condition while looking for info on the net for my own little slice of post-cancer heaven.
Just a simple google on "brown tongue" will get you some decent facts.
"Black tongue results from overgrowth of the filiform papillae on the dorsum of the tongue. The discolouration is due to the accumulation of keratin."
Since I'm not a doc, maybe there's someone out here that can help define that for us.

Jim