I recommend that you get in to see your doctor right away about this spot on your nipple. You should see either your doctors
">primary care doctor or a dermatologist
It is definitely possible that this black spot represents a 'blood blister' especially if it is painful and if you recently have a reason for friction to the skin. For example, blisters on the nipples is common in those who run or are otherwise very physically active.
More importantly, there is a chance that this black spot represents a mole. Moles are collections of pigmented cells under the skin. Although at the age of 21, it is still normal to have some new moles showing up on the body, there is always a small chance that a new mole is a early form of a serious skin cancer called a melanoma.
Your doctor can examine the spot for you. If they are concerned that this might be a suspicious mole, they will likely recommend that you have the mole removed so that it can be examined for evidence of cancer. Removing a mole is a simple procedure that can usually be done in your doctor's office with just some local numbing medicine injected into the skin.