I have a small circular bruise on my left calf just below the crevice behind my knee. It's very dark in appearance, almost black, but doesn't seem to be causing much discomfort. I don't really remember having injured this area. I am a healthy 24 year old female and am on birth control; the same type, Ortho Tri-Cyclen, for three years, and I suppose my main worry is the potential for this being a blood clot. Should I have worry in this instance? Do blood clots have a external appearance?