My front tooth was filled but it got stained with a Gentian Violet because I applied it on a blister.

Hi:It's best to go back to your dentist to evaluate the stain.  Most likely he will be able to remove the stain with simple polishing.

Go to your dentist, they may be able to polish it out. If it has gone deep into the filling your only other option will be to replace the filling.

Your first step would be to get your teeth cleaned and the filling polished. This should remove any topical stain you may have on the tooth. If it does not then the filling may have to be replaced because the filling is leaking and the Gentian Violet got in between the tooth and filling the same way bacteria can.