my son in law had a large cut on his leg in December and yesterday, he felt a bump approx 4 inches in diameter and he pushed on it and about 30 ccs of black jelly like fluid came out.  What would cause that?  I have tried to google it and cant find anything.
Your son-in-law might have had hematoma i.e. collection of blood after the cut in his leg. This collection of blood overtime turns reddish black which is called as chronic hematoma clinically. I suggest you to visit a nearby hospital and have the would checked for a sign of infection or any residual collection inside the cut. Thank you.