I shoot 45 Colt BP (the real stuff!) in my 73. Blowby is pretty bad (welcome to 45 Colt) but nothing overwhelming. It doesn't discolor the receiver or hurt the gun in any way. The brass carrier will be black, but it cleans up fine. I soak mine in heated Murphy's Mix (recipe is available in the BP forum) then scrub it with a toothbrush.
The one thing you may have to do while shooting is spray the tunnel & carrier with Ballistol at the loading table before every other stage - that keeps it from getting sticky & hard to move.
I did make a minor modification to my carrier. I filed a small chamfer on the front corners. Doing that leaves a little extra room for the BP residue that does get into the tunnel. Otherwise, welcome to the dark side!
Holler