Hey, I've just had this and didn't know it!
More than a month back I thought I'd dose myself with Corsodyl (UK brand, called Perio something in the US, I think) mouth wash as it treats gum problems - the main ingredient is chlorhexidine digluconate. Anyway it said on the label that it may cause non permanent staining of the tongue and sure enough mine turned yellow, then green and ultimately black.
Two weeks ago I had to get my mouth checked out and, needless to say, I supposed that the state of my tongue wouldn't look so good; so I stopped using Corsodyl. Fortunately, in the weeks since, the black went, then the green and now the yellow is going - you can actually scrape it off at this stage - eww! Funnily enough (or not, depending on your squeamishness) what remains does look kinda hairylike when viewed up close in a mirror. However, it's all beginning to brush off.
I really should have taken a picture of my tongue when it was at its blackest - with the light you probably wouldn't make out the yellowness now. I have to say I think it would be funny if Corsodyl were to put the following warning on the bottle...
Possible side effects:
Corsodyl can have side effects, but not everybody gets them.
*Temporary staining of the teeth.
*You may notice a change in taste or burning sensation. This usually goes away with continued use.
*May cause temporary black hairy tongue, mwa ha ha ha
At least the staining wasn't on the front half of my tongue - that
would have been embarassing. But I quess I'll one day be able to title my autobiography 'Mike Xxxx: the highs, the lows and my black hairy tongue.'
I didn't know what the heck it was either, so I, too, had to Google it. Definitely some scary stuff! 
