The only commercial loader of black powder cartridges that I know of is Goex and their line of "Black Dawge" cartridges.
Their website says ".357magnum, 146 grain bullet, loaded with black powder". I would assume it is a compressed load which should result in velocities equal to a healthy .38 special smokeless load.
"Cowboy load" usually means a light load so competitive shooters can shoot faster, and a lead bullet in order to be legal in SASS events. It does not mean loaded with black powder.
The best way to be certain that your ammo is loaded with black powder is to load them yourself.