Deaf: I wish I could give you the absolute specifics as to the thickness of the chamber walls on that Blackhawk where the notches are but I can give you the benefit of my experiences with those revolvers and after seeing them used and abused for a buncha years on the silly-wet range.
The benefit of my experience is that ya'll need not worry about those cylinders coming apart at all. I shot the snot outta mine with the heaviest loads I could brew up, as did a ton of other guys, and even saw a few that had been reamed out for longer cartridges - and all without any problems at all. I would have to say that I sincerely doubt that those notches mark any weakness in the Blackhawk cylinder structure at all. HTH. Mikey.