Sure enough.....

Mexican Ducks @ USGS Central Flyway ID

From that pic, it really looks like a black duck!! With the mallard/black hybridization that we get on the east coast, most of the blacks are not true blacks anymore and will have a small bar of white on the speculum like the ones pictured. Although, there are a few other differences. Bill and foot color, for example.

Very cool!! Looks like mallards, blacks, mottled, and mexicans are very close branches on the same tree.