"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.

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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.