Check out Flyanglersonline.com. Pull up FEATURES and click on HOME WATERS. The late Al Campbell ran a fly shop out there and has a lot of good stuff on the black hills area. I usually use this a a stop for a day or two to do the tourist thing with family on the way to Wyoming. I've only been there in Mid to late July. I wasn't too impressed with Frenchmen's Creek, did catch some nice Browns but there were stockers. You can walk in from a Horse camp down stream into a wilderness area (a Canyon). I tried to walk upstream from the lower end but I respect Buffalo, which got there first, too much to hang around.
Just off I-90 just before you enter Wyoming there is a small spring creek-(Sand Creek, not sure of name??) that some S.D.folks told me about that is supposed to be the best in the Hills area. Again check out Al Campbell's stuff on Flyanglersonline. Again to be honest, I really did not fish real hard, I was itching to get to the Mountains.