In any volume that is.
Of course,I strongly suggest one not try anything they are uncomfortable with.
Duplexing black with a small percentage of smokeless gives a much cleaner bore if your gun has a rough, unpolished interior.
This allows more shooting and less swabbing.
The practice is as old as the introduction of smokeless powder. Same for cartridge guns.
Muzzleloaders take a mixed (shaken) charge to keep reliable ignition,
cartridges are most easily loaded with the smokeless in the bottom of the case.
The unfair advantage it can give (all muzzleloaders are different, just like cartridge guns) is why it is
illegal in competitions, not because it is dangerous.
Very easy to spot a duplexed gun at a match. No shower of sparks dropping from the muzzle.
JT