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The book Historic Spots in California: Fifth Edition claims the term moro was used to indicate anything black, and
that tradition says that a lame black horse gave the name to this particular tract of land
The horse story is repeated in the Encyclopedia of California entry on Castroville, which also includes a possible reference to black soil of the area.
One other note, the last definition listed in the SpanishDictionary for moro:
- (animal) a. pinto
So the moro in Rancho Moro Cojo may have nothing to do with black, but translates directly to lame horse.
update
Trying to find more info on this 'lame horse' led to one more source on the moro designation in California Place Names: The Origin and Etymology of Current Geographical Names By Erwin Gustav Gudde
This source lists moro as referring to a roan horse 'of bluish color', and then defines Moro Cojo Slough near Portola as meaning 'Lame Blue Roan'. So not exactly a black horse, but dark at least.