But what about the strength of the convertible chambers in 45 acp and 9mm? do these chambers have some incredible level of allowable chamber pressure?
The short answer is...no.
And that is because of the wording, allowable
Any time you go beyond SAAMI specs, you are entirely on your own. Can you do it in a Ruger Blackhawk (either regular of convertible)? Sure you can. And you can most likely do it without damage to the gun, depending of course, on how much you exceed SAAMI specs. But if you do, it is entirely your responsibility. Not Ruger's.
Rugers are tanks, compared to Colt pattern SAs, but they are not without limits. You can blow one up, just like you can blow up anything.
The strength of the second cylinder is seldom asked about, mostly because very few people bother to load overpressure rounds for them. The 9mm in particular can't get much hotter (assuming a suitable powder) before running out of room in the case, and the .45ACP is seldom loaded hotter than standard either, mostly because anyone wishing more than these rounds give at normal pressures just slips in the cylinder for the larger round and shoots that instead. Also, for those of us who own other guns in 9mm or .45ACP, there is a risk of accidently putting "Ruger only" loads in them. Not a good idea for regular semi autos.
Unless there is some special heat treating involved in one cylinder and not the other (and for that, you better ask Ruger), cylinders with the same outside dimensions and basically the same size holes in them are going to have essentially the same strength.
Brass does play a part in total absolute strength, but not at regular working pressures. Cases properly supported by the chamber don't begin to let go until you get well above 60,000psi in rifles, and I expect about the same from handgun brass. The primary factors are basically the thickness of the steel in the cylinder (and frame), and any specific heat treating used.