I wouldn't feel comfortable with a .357 mag. if for some reason you were facing a charging black bear, especially a sow with cub, they will usually keep charging until the threat has left, or has been physically confronted and defeated.
But, and considering, black bears aren't generally very aggressive, it's rather unlikely you'll ever have to deal with an angry one. I've had hundreds of encounters with black bears, none have ever involved an angry charging bear though, and one was a sow with a cub. I thought she was gonna charge though, she came charging up on us, but thank God at the last second she stopped dead in her tracks, took in a big nostril full of human pee and feces, then she turned and ran, with cub in tow. I knew a guy that got one of his buttocks literally ripped off by a wounded black bear though, so it can happen, more so if they've been wounded or agitated I'm sure. And considering that one had been shot, and had been chased by dogs all day long, he was extremely agitated.
Some years back I would tease them with food, they would run up a tree, I would put more food on the ground, they would come down grab it, then go back up the tree. One day when I was doing this, "Big George" as my Son had named him, decided he wanted to go directly to the source, and tried to break into the truck with us. End of games, I started the truck and left "Big George" standing there on his back legs, he was what I would regard, as very agitated.
Personally, I would be more worried about a lion attacking, they can, and usually are far more aggressive than a black bear. I haven't had very many encounters with lions, maybe 2 or 3 in my entire life, but of those several encounters, one almost got me.
As for lions, I don't think I could hit one on the charge, they're fast, and also very stealthy, so you don't often know they are even there, until it's too late. Having been in a close encounter with a perusing lion, I don't think I would have been able to hit it while on the charge with a revolver, well at least not until it was on top of me. I think a short barreled 12 ga. is the best option there. But again, you're not nearly as likely to encounter a lion. But if you did get a good shot with a .357 mag., it should be plenty of cartridge to stop him fairly quickly I think.
GS