The .44 special Blackhawks are made on a medium sized frame, instead of the full-sized Blackhawk frame. It's the same sized frame as the old model .357 magnums that some folks were having converted to .44 special; just so you understand where the idea came from.
Smaller is handy for someone who doesn't want to shoot .44 magnums, and a lot of folks don't buy one or the other; they simply buy both, or several of both.
It's the same concept, on a larger sale, as buying a small, lightweight .38 revolver instead of a larger, heavier .357 magnum. A lot of folks prefer the lightweight, handy size of a .38 special revolver over it's larger cousin. I'd hate to have to carry a .357 magnum in order to shoot .38 specials, so I'm glad both are available.
Some folks even go to the trouble and expense of chambering handguns for .41 special, and commercial ammo isn't even available. Someone made a bunch of brass for it a while back, but I'm not sure if it's available any more, either. They have to trim .41 magnums to make the brass.
So why go to the trouble and expense? Mostly because it can be chambered in an OM Single Six. They like the bigger cartridge in a small, packable package.
Daryl