Simple answer, it doesn't exist, at least for model train smoke units.
The reason that the "smoke" is white is that it's really mineral oil vapor. To get black smoke, you have to burn something, and I suspect you don't want that happening inside your smoke unit!
Yep, that amounts to having additional particulates suspended in the air, and that may not bode well for one's respiratory system, especially those that have or are prone to asthma (most people with such afflictions can barely, if at all, tolerate regular toy train smoke as it is).
Plus, as the old saying goes, what goes up, must come down. Don't think I would be very keen on having black (or any other color) residue settling all over the trains & layout.
Truth is, if a steam engine is pumping out black smoke, that's from one or two reasons: 1) The fireman isn't keeping the locomotive fired properly, or 2) It's being done deliberately for run-bys for photo opportunities.
