I had a couple of similar spots, decided to taste them. It was not creosote, so I figured it was not from the lid. It was seep from the ribs, some type of coagulation of juices from the meat. I figure it is a reaction between the meat proteins, myoglobin, salt and smoke. Think smoke ring, but, since there is no actual meat, it takes on a very dark color.
See those dark spots. It was possible to knock them off, but, they didn't affect flavor at all.