I was on Doxy for 9 months and I have toth discoloration too.
My teeth are gray-ish, some are dark gray/blackish around the edges. GROSS!
I stopped the Doxy a long time ago and my teeth are the same. Once I get a job (I got laid off on halloween), I'll be able to afford a few visits to the dentist.
Have you tried a whitening mechanism like the white strips I see on TV?
Try not to worry, when you get better you can get those teeth of yours looking better than ever!
Peace and health to you,