Yup, by federal law black powder rifles, pistols, and shotguns are not firearms. In fact a lot of original and even replica guns - if originally designed before 1889 (IIRC) are categorized the same.
However, I believe that if such a gunare designed and built touse modern, smokeless power ammo (like most modern SAA clones) they are classified as modern firearms.
And as for the felon question - I believe that most black powder cartridge guns are considered 'bad'. I'm not sure if/where this is in writting or if there is just some assumed 'intent' for a felon to possess these.
I once saw a website where only BP guns were sold - and it had a long disclaimer on this 'felon' point. It doesn't apply to me so I didn't note the details.
I used to own a number of these muzzle loading revolvers back before I was old enough to own modern ones. And nope, This wasn't so long ago that they were all that was available anyway.