Have hematoma under knee cap, happened 18 days ago, whole knee cap hit asphalt lots of scabbing not healing fast. Now I have a blister above my scab, ?

Patellar bursitis. May be serous or sanguinous fluid. May need it drained. See your orthopedic surgeon.
Many possibilities. There are many types and causes of skin blisters. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause blisters on the skin. Have it evaluated by a professional and get the appropriate treatment.