The blue puppies I have seen, including my own, have an almost purplish brown hue to the entire coat (root to tip) when seen in good lighting. Compared to a black puppy, it is very noticeable. As a young puppy, you can identify them often by light whitish hairs in between the paw pads and sometimes on the underside of the ear. When next to a black puppy, you can tell them apart by the time they are 4 months old or so.
Sometimes black puppies will appear light in the muzzle after shaving because of the balding effect of the clipper (especially on bony structure) and the skin that is not suntanned yet. Before I had a blue puppy, I probably wouldn't have been able to pick one out and would have noticed something different about the puppy, but not know what it was. Now I can spot them across the ring easily. The picture really looks black. But pictures can be deceiving. Here's a picture I have posted before of my blue boy. See how his coat just doesn't look quite black, or it looks dusty almost and his face is light. Usually the hair above the eyes is lighter as well.