Blood blisters occur when the trauma to the skin ruptures small blood vessels, leading to a collection of blood under the skin. At first, this blood may be red in color, but as it ages and begins to be digested away by the body, it will turn dark purple or black in color.
Eventually, one of two things may occur. One possibility is that the blood will just gradually be digested away, as the body heals itself. The other is that a small hole in the skin overlying the blister will form, and the blood will simply drain away all at once. Either way, just because the blood seems gone doesn't mean that the blister is totally healed yet. You should avoid picking at the skin of the blister or peeling it away to keep it protected while healing continues.
If you notice that the area becomes painful or red, these could be signs of an infection, in which case you would need to go see your primary care doctor
for evaluation and potential treatment.
Also, if you have any more questions about this issue, or about any other general health condition, you can always talk with your primary care doctor at any time!