The evidence of being BLACK in origin is the appears throughout the lyrics as well as the way it is preformed.

usually any mention of "Sally" refers to a black girl or half black girl. She appears in many songs as a reoccurring character and is probably derived from Shallow Brown which is indeed a black shanty.

Not only that in most other songs, Sally Brown is outright called mulatto in the lyrics.

other evidence includes: "Sally got dredlong bangs" possibly referring to her genetically African hair. Overall the song screams black. It is very likely many of the old sailors were black among irish, english and other ethnic groups, and said song caught on and became popular amongst racial groups. There's even a Russian version of this song.