Let's try this one step at a time:
"If I understand it correctly, a faster burning powder shoul have less muzzle flash but probably more blast. Where the slower powders would have more flash and potentially less blast"
I guess there's probably a correlation between the two, but I have never noticed it. When I shoot, the only thing concerning me is where the bullet is going. We have lots of talk here about recoil and muzzle blast and I just don't get it. Yeah, if you're gonna shoot a sixteen inch .45-70 with a 405 over a max load you may experience at a tad bit of recoil and even, perhaps, a mite bit of muzzle flash. Like so what? What do you expect?? Geeze, these aren't rimfires we're dealing with. None of it makes a hill of beans if you're hauled down on the buck of your life or venison in the freezer. If it weren't for the dead deer, I wouldn't even know the rifle fired.
To me, Recoil and Muzzle Flash is just a bunch of targety poop. If the SOB nails your butt at the bench, take it away and shoot it the way it was supposed to.....off hand. I've found I've generated more flinches shooting off a bench than I ever thought possible. It ain't a natural position, and for me with a big load, it is damaging.
Pete