I agree with the 41 or 44 mags. The 454 and other big bores are another option. And if a semiauto pistol is legal a 10mm with the right ammo will work for bear without any problem.
I saw a show with Razor Dobbs where he shot not one but 2 cape buffalo with his 10mm pistol. And they're a heck of a lot bigger than a blackie.
Like Elkman says though you would almost want two for your purposes. Or you can go with a 6" barrel and practice, practice, practice. Not to hard to pack around and with practice one can shoot pretty well out to 75 yards or so with iron sights. I carry a 4" barreled Taurus lightweight Tracker loaded with cast bullets for bear defense.