Generally, in the midwest and northeast the only venomous snakes are rattlesnakes. They seem to generally stay away from areas where there are people. So, when I do trail hiking I don't usually worry about snakes.
When you go off-trail, there is a concern with the grasses hiding snakes. However, snakebites are a rare event here.
I looked up South Dakota snakes, and it turns out that the prairie rattlesnake only lives in open prairie land. So, I doubt they would live at the higher elevations in the Black Hills.