black duck, n.¹ — Green’s Dictionary of Slang

black duck n.¹

[SE black duck, any dark duck, e.g. mallard, redleg; like the birds, the Native Americans were considered prey by the 18C colonists]

a Native American Indian.

1767		T. Hutchinson Hist. of the Colony of Mass. Bay (1795) II 267: English soldiers and sometimes English hunters, [...] when they have killed an Indian, make their boast of having killed a black duck [DA].
1889		H. O’Reilly Fifty Years on the Trail 336: I lay on the ground with my glass, watching for ‘black ducks’.