Do you have a picture of the tattoo? Is it an actual tattoo (ink needled into the skin) or is it a stamp on the skin?
By law, primals cut from inspected carcasses need to carry the USDA mark. When the packer breaks a carcass with the intention of selling the primals (as opposed to further processing in house), each primal is stamped in blue (sometimes purplish-blue) with the USDA stamp from that house. (in the case of pork, each ham, belly, loin and shoulder receives a stamp). The plant I worked in would occasionally sell an entire head, and we had stamp each on on the forehead.
This ink is edible, although not necessarily desirable. It can look unappealing and can stain surfaces and meat that it comes in contact with.
Tattooing of livestock was a commonplace procedure done years ago, and while it is not common, it is still done by certain producers. It will be black in color, usually consist of numbers and/or letters, and will appear as several dots composing the letter/number.
Although there shouldn't be any harm in leaving this in place, again, it's not necessarily desirable.
The only way to remove either mark is to remove the skin containing said mark.