What Causes Blisters And Scabs On The Skin?

My fiance has recently had a "pimple" of sorts on his arm, that later turned to a black scab. More recently, he had an outbreak of several "pimples", on his back, that were sensitive to the touch. They have now also turned into black scabs. He has not done anything to them. He currently has five of these scabs on his back. Three are larger, approx 8-9mm, and two are smaller. What could this be?

He is 46 yrs old, 5"9" and 180lbs in good health