Gunslingers Gulch, 1858 Remington and Black Powder Times are all good. Black Powder Times is mine and its kind of slow but its still some good reading there.
When someone asks me, I point them to this forum. No more knowledgable people IMO. I'm an old guy but a blackpowder novice. Lot's of info right here to glean.
But there are (admitting this on an extreme level of sheepishness) There are several very good private groups on facebook . I'm in 8 of them including one for BP revolvers only, Sharps only (paper cutters and cartridge) Flintlocks only ML shotguns only and one that covers any firearm from the 1700-1800s. All of which not only have good reading, Q&A and tech...but also are sell and trade on the subject firearms themselves. And they are private groups you must request use of, so that cuts a lot of the foolishness out
But as I said, this site is 90% of my go to for all things cowboy and MLs