A black woman, probably. But is this a derogatory, or a "neutral" expression, and from what language or dialect? Is the term used from the point of view of a white man (Mick), or a black, darker/lighter skinned one (Mick's character in the song) maybe?

Negrita \Ne*gri"ta\, n. [Sp., blackish, fem. of negrito, dim. of
negro black.] (Zo["o]l.)
A blackish fish (Hypoplectrus nigricans), of the Sea-bass
family. It is a native of the West Indies and Florida.

There is a rum named Negrita.

mick referred to his wife bianca , as negritta...

Did anybody ever notice the similarities between the song Hey Negritta and one of the songs on Bo Diddley's album "Where it all began". I think the name of Bo's song was Hey Jerome! Ronnie Wood is credited as the inspiration for this song, but I think Ronnie must have listened to Bo's song for inspiration!

Ronnie has had a long and close association with Bo, so who knows. Hey Mona!

I ain't stupid, I'm just guitarded.

Bianca used the expression and Mick wrote the story around that.

It's what you mumble when you're going south on a babe...