Green’s Dictionary of Slang

1. (Aus./NZ) a black person in a sexual context .

1907		N.Z. Truth 26 Jan. 6/4: [heading] Some Women’s Craze for Black Stuff.
1914		Sport (Adelaide) 21 Feb. 10/2: They Say [...] That Cockeral seems hard up for a tart, see he takes on a bit of black stuff. He does not draw the colour line.

2. (S.Afr.) black labourers.

1937		C.R. Prance Tante Rebella and her Friends (1951) 60: They still kept ‘Father’s Folly’ as a private reserve for their Kaffir labour-tenants – ‘black stuff’ which can always eke out a monkey-brand subsistence.
1941		C.R. Prance Tante Rebella and her Friends (1951) 151: There is shark’s small-fry for sale to ‘black stuff’ as ‘sardines’.