The blackest paint I have seen is Benjamin Moore's INT. RM 12 Black. In the center is a 0 0 0 black for reference. The closest to that I have seen with paint is the INT. RM 12 at 46 46 46.
I know EXT. RM 4 Black Forest Green looks blacker, but it really isn't. It leans green, hence the name.
This stuff is pretty dark, but there is a way to deaden it up even more. It won't make it darker, but definitely makes it reflect less light...
The problem with paint, even flat paint is it reflects light to some degree, even flat black. There is a way to deaden flat black even more (only from a regular can though, this won’t work with spray cans). Add 4 tablespoons of cornstarch per oz. of warm water. Then 4 tablespoons of the cornstarch slurry per 2 oz. of Flat Black paint. Believe it or not this will deaden the flat black down a lot.
It sounds crazy, but is really works. So to make a quart of this black hole black you would need 24 ounces of the corn starch slurry and 12 ounces of the blackest black paint you can get and in the flattest finish possible. That will give you a bit more than a quart (36 ounces) but close enough.
The only down side is flat paints show wear and dirt easy, the plus is you can reapply a coat and it's a dead black again. I'm not sure I would do this if you want your speakers to look 'pretty', they definitely won't have any shine or have that 'piano' finish look, but it doesn't sound like this is what some are looking for anyway.
Anyway, it's worth checking into for those that don't want to use fabrics.