American Black Duck is similar with broader brown edgings to upperparts and a bluer speculum. Female, immature and eclipse male Mallards always have blue speculums with white borders to both the front and rear margins. Beware of hybrid Mallard x Black Ducks that are usually paler than Black Ducks, with blue speculums and usually with white borders on one or both edges of the speculum.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.