It depends on the breed and how solid black lines were originally developed, as to what might be expected regarding underlying genotypes. Fred Jeffrey books have many old breeder tales. Most exhibition black lines were s+ based (Pekins, Orpingtons, Australorps, OEGB, etc). Breeders believed some sort of bronzing /red modifier enhanced green sheen, a balancing act.
But R. Okimoto's commercial Australorp line was S silver based, has green sheen. Personally, I believe the opposite, it's much easier to hide red leakage on black than silver, and any white on a black is considered a flaw. But a silver feather looks closer to blue, and brassiness a worse flaw in blues than white. But, the ideal world would be no leakage in the first place (although I know some use it in yellow legged black pullet breeding lines, the breeder males having some leakage).
Early Black Wyandotte lines were developed differently, some selected from overmelanised silver laced. These black lines occasionally threw silver laced.
Smyth has mentioned that Ml is very good in melanising both ER and eb hens, but not as good with roosters, need additional melanisers.
Then there is Smyth's research line, eb based blacks. It was supposed to have multiple melanisers, including Ml. Now you would think if both Pg-Ml present, the hens would have double lacing, not solid black. In a cross of this line to a non laced blue rooster, they got laced blues. So the eb black line had Pg. Must have been the additional melanisers that gave sold black eb hens.