What Could Be The Blood Blister On Right Side Of Knee?

I'm 31 yrs old, 174 lb. 5'11" tall. I had what I thought was a blood blister on the right side of my right knee which burst on its own but now there's a deep hole in the center of it developing. It has been about a month since it has developed but only 3 to 4 days since it burst and the hole started forming. I don't think it's just a blood blister anymore. Any ideas as to what it may be?