Sorry for the long post. Everything that I have been able to find and come across mentions Fibroma. I actually did a search on google and the first thing to pop up was some information from Huntingnet. The other information is from a biologist. I don't think I would let if spoil my hunt...I just simply wouldn't shoot that deer. haha

From Huntingnet: Skin Tumors
Hunters sometimes shoot a deer with lumps on the skin. These bumps are usually non-cancerous tumors called fibromas or papillomas. Viruses cause both. The only difference is that fibromas grow either from the skin or from the layer beneath, while papillomas grow only from the skin. Both lumps are dark-colored and hard to the touch. These growths generally do not cause problems for deer, but people are curious about them. Hunters may feel venison is inedible because of these tumors. Since lumps are usually found only in the skin, not in the muscles, there is generally no problem. But tumors located inside a deer's body might be cancerous and the venison should not be eaten.

Ask the Biologist

• Last year I harvested a deer that had several hard black growths in and on its skin. What causes this?

What you saw is called a "fibroma" or "papilloma". It is a hairless external black or grey tumor that is caused by a virus transmitted into the skin of the deer when it is bitten by an insect, most commonly a deer fly. They are attached to the surface of the skin and are generally not a problem for the deer unless they become infected (rare) or become so numerous that they interfere with the deer's vision, locomotion or eating.
Fibromas are not contagious and cannot be transfered to humans or pets. While grotesque and unsightly, fibromas also do not affect the meat and should generally just be cut off at the surface of the skin and discarded before butching the deer. The attachment of the fibroma often recedes and I've seen them in late spring where they appear to be hanging by a stalk. Interestingly, the virus that causes cutaneous fibromas appear to be specfic to deer and are not observed in cattle, horses or other livestock.
Don't let it spoil your deer hunt !