Ring-billed Gull melanistic adult with normal Ring-billeds

A strange-looking beast, the size and shape of surrounding Ring-bileds with the same leg color, yellow iris, and ringed bill. Primary pattern when seen briefly flapping was like Ring-billed. Mantle tone was like Ring-billeds, too. Not thought to be soiled, as even the underwing coverts were dark when the bird flapped. Tail was medium gray.