The Rodeo is rated for standard factory ammunition, right? If so, black powder shouldn't be a problem, assuming of course that it's loaded as black powder cartridges should be.
Smokeless powder tends to run higher pressures than black powder. Since standard factory loads are smokeless, a revolver that's rated for them should be OK for black powder too.
You have to be careful, though, because black powder cartridges aren't loaded using the same tools and rules as smokeless. If you're going to roll your own, make sure you've studied up and know what you're doing before you start.
You don't need a black powder frame to shoot black powder. The reason it's called that is that Colt switched from the old style frame (with a set screw you had to unscrew before you could withdraw the base pin) to the new style (with a spring-loaded plunger you pushed sideways to allow yourself to withdraw the base pin) at about BUT NOT QUITE the time they stopped saying the Single Action Army was for black powder only and started saying you could use smokeless loads in it too. Either frame design, in modern steel, is strong enough for standard smokeless loads. But in the original Colts, all those with the "black powder" frame were made with steel that is probably too weak for smokeless. Most of those with the newer type of frame were made of more modern steel, but not all of them. Once again, as with anything firearm-ish, when you're dealing with one of the old Colts you have to know what you're doing.