I don't think you guys are gonna find a true black... Most likely the darkest/'blackest' tint will be smoke (Rembrandt/T-Visor sells a nice smoke one), or the blue/purple type that BM sells. I know that when shopping for fabric dyes, they all have a seperate color for the base. Like when a black t-shirt gets old and faded, it has either a brownish red tint for a red base, a greenish tint for green base, and so forth. The ones that look the most 'black' when they fade are the blue based ones, imo. Not sure if the dye that you are using for the visors follows the same pattern or not, but its just something that I've noticed that seems to be in accord with each other.
This is a BM visor from the outside:

And this is a Rembrandt/T-Visor from the outside:
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