Visiting Richmond and the James River on Sunday, February 5th, Phil Kenny and I looked through a couple of thousand gulls near the Brown's Island footbridge. We did not see any of the reported rare gulls, but in the process of looking ran across some abherrant specimens. This first one is a Herring Gull or larger sized bird, with dark slaty gray mantle-scaps-coverts (I upped the photo contrast a little, which I think makes the bird look darker here than in person). Not dark enough for a Black-backed Gull of either ilk, but much darker than the surrounding Herring Gulls. The feet were pink, iris pale yellow, orbital ring yellow-orange. There was a red gonydeal spot, with a hint of a black band left on the bill. It would have been fun to turn this into a Western Gull, but I found pictures in Howell and Dunn's Gulls of the Americas of a Herring Gull x Great Black-backed Gull hybrid that look identical to this bird. Note the comparison of the darkness of the gray with the Herring Gull in the background. (Apologies to anyone looking at this on an Apple product and seeing the pics sideways)
During the same visit to Richmond, we also turned up this bird. The pictures are not good, but this is a medium-sized gull that is almost a uniform gray all over - back, breast, head, sides ... gray. Primaries are black and almost entirely lack white tips. Again, all excited about what this could be. However, it was the same size and shape as the surrounding Ring-billed Gulls. Its iris was pale yellow-white, legs were yellow, and the bill was black tipped. So except for the feather coloring, it matched the characteristics of a mature Ring-billed Gull. So my assumption is that is what it is, but with some form of melanism that affected the feather coloring, making pretty much the whole bird the color of the normal back feathering.