If you lost 4+ quarts of oil in 40 miles and what was left was "solid black" I would imagine you have a pretty serious problem with oil getting into the combustion chambers and then the combustion being blown past the rings... the cause of the black oil (?). Chances are your compression test is false because of oil in the cylinders sealing bad rings.
I'd start by pulling the plugs and seeing what they have to say about the problem, but I wouldn't expect any good news.
