I have done it. It is rather painful to tatoo over a scar, but it does work
I would not try it my self, it seems that it would be pretty dangerous covering them with the tattoo ink, it could cause an infection even. To be honest, I would talk to a dermatologist about options instead.
I have had a few tattoos placed to cover scars that I've got & the only thing I can say is the normal pain of a tattoo is nothing compared to tattooing a scar... it hurts like hell. A lot of tattoo places won't even try because scar tissue is unpredictable & for some tattoos great, but for others it doesn't take properly & looks worse then the scars themselves.
I would try a dermatologist for options & try scar removal options. Bio-Oil may help lighten it.
Melissa