Duplexing for ML with Black is to ignite the smokeless powder - not recommended by anyone.
Incorrect.
Duplexing in a smokeless build does not use black or subs at all. Its 2 smokeless powders. A small amount of a faster powder and larger amount of a slower powder. Its EXTREMELY common.
There are tons of traces on Dougs board of duplex SML loads.
If all duplexing is banned in ML competitions you may want to look at the Pyrodex pellets then. That black igniter end sure appears to be real black powder.