(edited for brevity) I have a lot more faith in Nosler Partitions than I do in any shotshell load. Away from the house.... .44 Ruger or S&W revolver.
Shotshell, I agree. Buckshot, good, maybe. Hard-cast slugs, yes, very good bear "medicine". Brenneke slugs have a good reputation.
To the O.P., what size are your black bears? For some of the larger ones, .357 might be a bit light, but hard cast, flat point slugs work well for heavily muscled animals.
As has been said before, a long gun is hands down superior to a handgun and spray is actually more effective (easier to aim and to hit with, as the stream can be directed even after pressing the "go" button) and across the Northern Tier of this continent has a pretty good track record against Brown, Grizzly Black and Polar bears at keeping humans injury-free. Better than firearms, actually. Google Smith & Herrero for some studies published in the Wildlfe Journals.
If a long gun is inconvenient to have at your fingertips, 357 Magnum is a bit lightweight for bears over about 150 lbs. Beyond that, something that starts with a .4 and weighs over 240 grains. 1500 ft-lbs of energy would be comforting, but hard to come by in anything smaller than 41 Magnum.
My advice, keep the spray. Buy an extra can and use one whose expiration date is near or just passed for practice (less than a box of GOOD ammunition). Once you have seen how easy it is to spray a target at 10 yards, you will probably have more confidence in it.
In Alaska, if you shoot a bear in Defense of Life or Property (DLP), you have to report it to the State, preserve the hide and skull, turn everything over to the State. You go to all the trouble and you don't even get to keep the cape! (Though you can keep any pictures you take.) Then prove to the investigators that the shooting was necessary or face charges. In Alaska, the presumption is on the side of the human, but in your State, I don't know.
If you WOUND the bear, you are morally (if not legally) responsible for the damage the bear causes. (In my opinion) If you successfully spray a bear, you have taught a bear that people are to be avoided. Pretty much win-win.
Good luck,
Lost Sheep
Lost Sheep