It can be done.... some pics:
Tattoos to cover Scars
Here is some info I got:
Although many scars have been successfully covered with a tattoo, keep in mind that many of these procedures were tedious and in some cases, extremely painful.
Decide if you should cover it. The scar should be at least 1 year old before even considering covering it with a tattoo. The older the scar, the better and the more likely the area will take ink.
Scar tattooing can be even more painful than regular tattooing, because scar tissue is more sensitive than regular skin. Unless the nerve endings were destroyed during the ordeal that left the scar, you will feel pain more acutely in scarred areas. Plus, if the tattooist has to go over the same spot several times to get the skin to take ink, each new pass will become more painful than the last.
Talk to your Dr(dermatologist) first, there may be medical reason for which you may not tattoo over your scar.
The tattoo also will not change the texture of the skin in scars that are raised, so there may be a noticeable difference in the tattoo design. These abnormalities may leave you with poor results that in the end you will not be happy with.
Areas where skin grafts have been done and large burns may not take ink, and you don't want to start a large piece to learn partway through it that your skin absolutely will not take ink.
Keloid scars, especially severe ones, are rarely tattooed over.