I've bought BP guns through the mail from Cabela's, and everything I bought had serial numbers. The Civil War Colt and Remington revolvers had serial numbers. Pretty much anything factory-made, except some really cheap suicide specials, has had serial numbers for a LONG time, going back to the 19th Century.
Even if a black powder gun is not a firearm under Federal law, it still makes sense to give it a serial number for all the other reasons. Something to write down in your "valuables" book as a precaution in case of theft. Something to tell you whether a product recall or manufacturing problem applies to a gun you have or are considering. That kind of stuff.