Saw one in NY last week that dark mousing in a field as I was driving home from Gander Mountain. My buddy mounted a coyote he shot with the bow. That bugger was real dark and showed no signs of coydog at all. Not as big as the one in your picture, though.
Could be a cross, or, just a darker genetic trait and a hefty prime winter pelt!
Nice critter though, make a hell of a mount!