I always associate black powder with side locks and underhammers, percussion caps and flint, muzzle loader has lately come to be associated with modern inline, shotgun primer and pellet loads. I have zero knowledge of inlines, except they are legal and you can buy them and the substitute powder at Bass Pro. And that there are many sales people that don't know the difference between black powder and a substitute.
Black powder, Goex is my powder of choice, (hard to find locally and not sold at any big stores- they only sell substitutes)
Mexico, Mo. https://www.grafs.com/retail/catalog/category/categoryId/3501
Clarksville, Ar. https://powderinc.com/black-powder/
Davide Pedersoli, Lyman, Aldo Uberti all make quality replica black powder guns (probably any of the Italian makers do, but some I haven't seen) Thompson Center used to make good percussion rifles but they don't list them on their site at present, my rifles are T/C Hawken a .50 and a .54- the .50 is easier to carry, easier to shoot and uses less powder and less lead than the .54, but I still like the .54 better. I have owned CVA and Traditions units that were more cheaply made but functioned fine. Mechanically there's probably not a great difference from brand to brand, price is based on grade of wood and finish, my Traditions revolver was made by Pedersoli.
https://www.dixiegunworks.com/
https://www.trackofthewolf.com/List/Category.aspx/485
http://www.longrifles-pr.com/index.shtml
https://taylorsfirearms.com/long-guns/blackpowder-rifles.html