One other question - will a 209 in a Knight "red disk" light the Blackhorn reliably - if I switch I want the Huntsman and Knight on the same powder if at all possible
If this is a DISC Extreme or Elite, then Yes, the Red Full Plastic Jacket will work, as long as you use a good strong 209 primer, such as a Federal 209A or CCI 209M (Magnum Primer). Use a new FPJ for every shot, for best results.
If this is a Plunger Type rifle such as the MK-85, BK-92, LK-93, Wolverine, Bighorn, with the Red FPJ Conversion, then I would not recommend shooting Blackhorn 209. It can be unsafe it these type rifles, unless charges are kept down away from max.
Blackhorn 209 works best in actions that lock up, such as locking bolt actions, drop block, and tip up (break open) style rifles where the primer cannot move away from the primer pocket/breech plug.
Blackhorn 209 is not reliable in the orange primer carriers for the Huntsman, you need an aftermarket or custom breech plug for best results.