I own & shoot black powder muzzle loading rifles & cannons. I also own a reproduction Spencer rifle & have shot it. So I am familiar with handling period weaponry. For decades, I have participated in an annual program at Stones River NB about Wilder's Lightening Brigade's at Hoover's Gap on the first day of the Tullahoma Campaign 24 June 1863. Our programs routinely include our 1841 six pounders standing in for Lilly's jackass mountain howitzers. Our friends from other NP battlefields provide the 3" ordinance rifles. A group of mounted reenactors do a demo of both Henry & Spencer repeaters.
We have had a long list of historians & experts on Civil War weaponry speak on the topic of the Spencer repeater. I have written a short lecture for explaining the Spencer to visitors at the park. A comrade who does Spencer & Henry repeater programs for parks, CW Roundtables, etc, loaded black powder cartridges just to see what firing 50 rounds would be like. Other than the stinking black mess you would expect from cleaning any black powder weapon, he encountered no problems.
I have, on two occasions, been Ed Bearss "local guy" who scouted the route he intended to take his bus tour. The existing section of the historic Hoover's Gap road is a toe curling experience from the front seat of a tour bus. His word picture of the Hoover's Gap action vividly brought the first time, men armed with Spencers confronted men armed with muzzleloaders to life for me.
The dozens of first person accounts that I have on file from Wilder's Brigade veterans all contain at least one reference to how reliable the Spencer was. 1v1, 1v3, 1v4 it didn't matter, they always believed they could achieve fire superiority in any situation. My opinion of the effectiveness of repeaters in the Civil War is based entirely on their experiences.
So, the short answer is, 'Yes, I do realize that repeating arms cartridges contained black powder because I have decades of hands on historic weapons experience & have been taught by scholars who generously shared their hard earned expertise with me.'