Both times when Chuck made a face on the volleyball in blood, it never got washed away by the rain. Maybe the first time he put it in the cave, but the second time he had it with him on the raft, and water was going all over it!
It's been 6 years that Wilson has had a print on him. I think its obvious that Chuck made the print again more than once and dried blood will stay after it has been imprinted again and again over a long period of time.