I have a blue heeler mix and she looks very similar. I am almost sure there is some blue heeler in this one. Mine is more spotted than marled like some blues, its a more uncommon coat. Dalmations are one of the breeds that were crossed to create the blue heeler so that may account for the coat like a dalmation, but this dog has a more stocky body and the shape of the head is much more like a blue, also.

I've never been able to figure out mine either. She's a blue/?