The Black seducer pattern is a great looking stonefly interpretation. I have also had good luck swinging these after dark. These are basically a really pretty stimulator pattern with an extra black wing case and some flash tied in. You'll love these high quality flies.
Look at this beauty! These flies work best for me just before and after dark. However they do a good job of imitating some dark colored stoneflies. They will certainly work all day long if conditions are right.