Broadly speaking, black bears in the US are nowhere near as aggressive as browns, and shouldn't cause great concern when encountered - just a healthy respect and a wide berth. There are only a few areas in the country still populated with browns, mostly along the Canadian border and in the Yellowstone area. Canada/Alaska still has a ton of browns, along with some very big and much more aggressive blacks. Bergmann's Rule, which states that similar warm-blooded animals grow larger the further from the equator they're found, could have the bear as it's poster child.
Present day, the only hotspots of real black bear viciousness in the US (away from the Canadian border) are found in the Southwest (AZ, UT, NM, CO) and in the Great Smokies in Tennessee. Several attacks and fatalities have occured in the Smokies over the past 20 years, which is generally blamed on the bears having lost their fear of humans due to the hordes of people that visit the park annually (by far, the most visited national park in the system). I've always wondered why the results of bear/human interaction in the Smokies have been so different than in Sierra clusters (Yosemite, Mammoth, Whitney Portal), where bears and people mingle almost as thickly. Cali bears with a laid-back Cali attitude?
