Believe it or not, dogs can get acne. It is more common in certain breeds too. Boxers get them a lot on their chins, Greyhounds get them on their chests, etc.
Like human acne, it means a pore got clogged with bacteria. A few pimples here and there is nothing to worry about, but if they are itchy and your dog is scratching them, he could get a secondary bacterial infection of the skin.
If they are large or filled with fluid, I would have your vet take a look at them. Blackheads are called comedones, if you wanted to look it up for more info.