The plot thickens...
I got a reply from the guys at the U. of Arizona. They seem quite confident that it isn't a digital artefact and it really IS a very dark patch. They see the signature of Mars atmosphere above, well, nothing.
Whether it's a cave or an unusual black fill of the crater is still open for discussion, but perhaps I should eat a few slices of humble pie for my assertation that it "must be" a digital artefact.