It looks like the top layer has been picked open. Do this now:
Because there is a fairly large dark area around it and the center is open, arrange an in-person medical assessment within 24 hours, especially if this began as a blister, bite, boil, burn, or unexplained sore.
Get urgent care today if you develop spreading redness, warmth, worsening pain or swelling, pus/bad smell, fever, red streaks up the leg, numbness, the dark area is enlarging, or you have diabetes, poor circulation, or reduced immunity. Those can be signs of infection or a deeper skin problem, as described in
AAD infection warning signs.