They mature throughout the summer and become sexually active adults the following fall, usually in November. They are an important part of the complex of organisms that break down leaf and other organic matter on the forest floor. They are soil builders. They are not harmful to people or pets. Probably what's going on with the black widow you saw was that it was still a juvenile.
Black widows start out life as small white spiders with patches of yellow.
As they molt and grow the patches change color and expand in size until they merge together. The yellow color first darkens to brown, then red, and finally black. Males, however, mature at a much smaller size than females, and retain their juvenile color pattern as adults. But, like the females they also have the red hourglass underneaththeir abdomen. The black widow you saw is a female only one molt away from adulthood, so she still retains some juvenile coloration.