"Recessive Black" gene mentioned in the following:

Jeffrey, F.P. (1988) Crow wing and recessive black in Old English Game bantams. ABA Yearbook, Amherst, MA.

Jeffrey, F.P. & Richardson, W. (1995) Old English Game Bantams as Bred and Shown in the United States. ABA.

* I haven't read the first one (American Bantam Association article). It should be enlightening.

There is also Smyth's eb based Massachusetts Recessive Black Line (a research line), but this is an accumulation of melanisers (including Pg-Ml), not a single gene.