Eghad,
Recently I've read some posts to the effect that the .45 Colt's problem with blow-by and weak brass have more to do with oversized chambers than with any problem in the case design itself. And this does make sense. Some of the .45 Colt brass out there is as thick as anything going. Also, the .45 is a simple, straight-sided case. If that design was prone to gunk blowing back along the sides, it should happen with other cartridges such as .44 Mag, .357, and so on.
It's hard to pin down exact specs for the .45 Colt. That's part of the problem. For example, there's bore diameter. Original was .454, but for decades now makers (perhaps all makers) have been using .452, to match what the .45 Auto uses.
Then there are case rims. As I understand it, the rim on the .45 Colt is more pronounced than on original versions. On the other hand, for a little while around 1910 or so, the US Army used a wide-rimmed version in a double-action revolver; supposedly the rim was wide enough that you couldn't load six in a Single Action Army, only three, leaving every second chamber empty. It was probably just as well that the 1909 Army cartridge's design discouraged people from using it in SAAs; it was smokeless, and loaded rather hot.
The Colt's been around since before standards were that much of a big deal. So it has trouble meeting them.
I would think the .44-40 would be the better cartridge for black powder loads or CAS, yes. I would think so, although the .44-40 does have some problems of its own. Unfortunately, .44-40 ammo is "rare" and expensive, at least compared to .45 Colt.