Years ago when I was on Doxy my teeth turned dark grey, almost black and it was when I stopped the Doxy that my teeth returned to it's normal color. To this day I have no idea why the Doxy did that but it did.
Today I used a tooth powder called Good Gums (I am not connnected with this company in any way) and it really helps whiten the teeth. It's all natural and has great ingredients!
http://store.liberationwellness.com/products/Good%252dGums%C2%AE-All-Natural-Tooth-Powder.html
Hope this helps,
Denise