#2). The "biggie" is that the oldest model blackhawks were copies of the colt with coil springs... Point: You could bang the hammer down on a live round/primer and it would go off -- so you loaded 5 and let the hammer down on an empty chamber (unless you were in town and expected trouble...). Some boob didn't know this, loaded 6, shot hissself, SUED... ruger --in the settlement-- came out with the "new model" which has a "hammer block" so you cannot do this... And offerred to modify guns returned to the factory... So yours is a little safer. Thinking of a gun as anything but "extremely dangerous" is bad attitude. You manage the "dangerous" for your purposes, but thinking of a gun that is not completely disassembled as "safe" is "stupid" MUCH.
#1). The "old" .45's from the 1800s used a .454 bullet. I think this survived up until WW II. At the same time the ".45 ACP" (automatic Colt Pistol) of government adoption, fame, use/ employed a bullet --jacketed--.451 or .452... At one point, Colt discontinued the "single action." [Ruger SOLD Blackhawks, couldn't make them fast enough.] Colt reintroduced the single action, but used the same barrels as in ACP autos and revolvers... .452... Your gun is probably .452. NO BIG DEAL. Lead "swages" /"squeezes" down very easily. If the loaded round will chamber you should be fine. If your gun is exceptionally "tight" then this slightly larger bullet might just make the round too big to chamber in the cylinder. MIGHT. probably not.
.45 Long Colt works at pressures under 25K CUP. Often in the 12K to 15K range. (Like shotguns.) This might raise pressure enough you could measure. Might not. (.44 Mag works at approx. 40K CUP). Nothing to sweat over.
This is handloading. You have to make up your recipe and try it. If it works for you, you stick with it. If not, or if you want to try something else, you change. This is a huge case meant to hold A LOT of black powder. For cheap, today, you use bullseye or other fast powder in tiny amounts. HS6 is a fine "medium" rate powder. And for max perrformance, 296 or 110 gets the most you can... (or 4227 or 2400 or
) HAPPY HOLIDAYS. Luck. Happy trails.