The picture is of a blood blister on the first finger of the left hand. (This particular blood blister was caused by an accident with some needle nosed pliers, whilst constructing something from IKEA without properly reading the assembly instructions).
Following advice found on the internet, that the wound would be more sterile if not punctured, the fingers owner did not attempt to pucture the blister.
There is a small weakness visible in the bottom left of the blister, and after a few days, the blister gently burst at this point and the very dark blood seeped out.