1998 Chevy Blazer Tranny slips in reverse, fluid is dark re

I would like your advise on how to fix my transmission. It slips in reverse, seems to shift ok though. The fluid is darker than usual but does not smell burnt. The 4 wheel drive works fine and it shifts through all the gears normally, just wants to slip in reverse. I was wondering if I should change the fluid and filter or put some kind of additive in it or if you think this is something more serious?