According to UPS's tarriffs and terms of service, the company defines firearms in accordance with Title 18 Chapter 44, just like the Post Office does. There aren't any special rules for black powder guns, just like with the Post Office.
But, just like the Post Office, you have to get past the "gatekeeper" who may not understand the rules. I went through something similar at the Post Office when nobody, not even the postmaster, had a clear handle on how to ship a firearm. Fortunately, they were willing to learn and review the regulations with me, so it worked out.
My advice? Ship your black powder revolver with UPS if you want. Personally, I'd go with USPS because Priority Mail is faster, cheaper and extremely reliable.
Here is a link to the relevant rules: http://www.ups.com/media/en/terms_service_us.pdf