Because black bears are intelligent and have distinct personalities (at least as much as dogs), they are somewhat difficult to generalize about. One trait they all seem to share is they are always hungry and always interested in food. So, it is a good rule to always safeguard your food.
Another good rule I heard from a hiker who frequents the Sierras in CA, where the bears are not very afraid of humans, is that once a bear has successfully grabbed your food, it is not your food any more; it is his food and he will defend it on that basis.
Apart from food issues, the next most common incident between bears and people are accidental meetings, where neither the bear nor the hiker expected to see one another. Generally these end with the bear turning tail and scampering off - but not always.
When the bear doesn't leave quickly, it will commonly be because it is a mother with a cub nearby, or it has no good path of escape. In either case, you've got a testy bear who wants you gone. The best thing to do then is go away, slowly and deliberately, back the way you came and wait a while before proceeding again.
I am also an advocate of talking to the bear, explaining what you are doing in a calm, friendly voice. Experts may not think this is worthwhile, but I do. As I said, bears are intelligent creatures.
Good luck. And don't worry too much. 99.9% of bear encounters end safely.