Generally black smoke is unburned fuel. Intermittent black smoke as you shift does indicate some problem, though perhaps minor, in engine tune or built up carbon coming loose. The fact that it only occurs when shifting is reason to have some relief. I'm sure other's more knowledgeable as myself will be able to address this more fully. It's possible that your initial timing is a bit retarded. Do you see the same puff of black smoke if you just accelerate engine speed with no load (stopped)?