I don't think you need a "special scope" for blackpowder, although I agree that you should not put a cheap or
rimfire scope on one. I think that scopes for shotguns and muzzleloaders
are not necessarily designed to handle
more shock, but instead have different parallaxes (75yds.), different magnifications (2x7), and different reticles.

Some of it seems like marketing to me, and I don't think you can go wrong with a basic 2X7 or 3x9 from one of
the better brands like Nikon, Leupold, Burris, Weaver, etc.

I put a basic Nikon Pro-Staff 3x9x40 ($150) on my ML and I haven't had any problems.