Usually a purple bruise will start to change colour in a few days, and turn brownish/yellowish. If this 'circle' doesn't do that, then it's unlikely to be a bruise.
Is there a feeling of a lump or raised area?
Anyway the best thing may be to keep an eye on this bruised looking area of skin. If in a few days it is still purplish blue, then get it checked out with a vet.
Also don't hesitate to get your dog to a vet if he begins to seem unwell in any way. Dogs are very stoical, will usually behave quite normally unless they are in really bad pain/discomfort. Then it might show as a reluctance to eat/ a reluctance to go out or go for a walk/walking carefully/wanting to sleep more, and looking generally 'down' etc.