I'd add that my experience suggests Maine black ducks respond better to a carefully placed spread of a few (3-7--and superstition says it MUST be an odd number) decoys rather than a larger spread. And mallards are not shy at all about dropping into black duck decoys, but black ducks can be shy about the opposite.
The Maine saltwater standard is 3-5 oversized black duck decoys. This is where those LL Bean magnum decoys were developed, and they still work.
Also, other than the dawn flurry, by far our most effective technique is to note where ducks get up when disturbed, and quickly set up on EXACTLY that spot. I don't know if it's the same ducks coming back, or just that birds are up and moving and favor the same spots, but it's amazing how often blacks are back in the decoys within 15-20 minutes. This is much more true of small groups of birds than of big flocks. When I am seeing big flocks bunched together, the hunting is always tough. I much prefer scattered small groups here and there and on the move.