Anyone who thinks that a black powder revolver is not a FIREARM as it applies to concealed carry or on public transportation is seriously mistaken.
You WILL be arrested or cited if you're carrying it concealed without a permit, or if you check it in with baggage without declaring it.
And really, what's the point of NOT declaring it?
This foolish semantics of "It's notttttt a firearmmmm" with a sly grin won't keep you from appearing before a judge.
If you want to carry a black powder revolver concealed, GET A CONCEALED WEAPON PERMIT.
If you have it in your hard-cased, locked luggage, DECLARE IT.
Or perhaps you'd prefer to call Willie to post your bail when you're sitting in the cell.
As for exterior idenification of which luggage contains a firearm. This USED to be true, in the 1970s and up to the early 1980s, as I recall. It was a red tag on an elastic string, attached to the handle. I used to grumble about it, pointing out the obvious invitation to steal, but didn't make a fuss. Even then, you could find yourself in an interview room for just raising your voice.
I believe it was about the mid 1980s that they moved the tag inside the bag; took years to be sensible.
I've never had an airline refuse my baggage if the firearm it contained met all storage requirements. Some airlines do, however, but from what I've seen they tend to be local and on the East Coast.
I'm a Westerner. I don't go any farther east of the Mississippi than the Interstate Barbecue in Memphis.