Suggest Remedy For Blood Blisters And Bruising On Knee

My 93 year old dad has developed a one inch black bruise behind his knee and it appears to have a lighter part pinkish cream in middle that is swollen like a blood blister inside of the dark bruise. It is raised and slightly firm but yet soft to touch. What could that be since it does not bother him at all. His legs are thin now since taking aldactone and furosemide each morning. Thank you.