I was one of the guys on the road side of Miller's yesterday......there was a very heavy black fly hatch (or spinner fall, not really sure how the adult life cycle goes) that went on for several hours that the fish were keyed into. I had my best day of the year numbers wise (not sure how many I caught, lost count) I did have a tough hour or so where I was having trouble getting consistent hookups but other than that the action was pretty steady.
I have had great results the last couple of trips with a soft hackle that I believe represents hatching black fly pupa, if you do some research you will find the pupa rocket to the surface enclosed in a bubble of air. You also get to use some of that opal tinsel! The fly has a body of mirage flash and a brown partridge hackle. I tie the body short on a size 18 hook and fish it on the swing. You just want a dot of flash enclosed by the hackle. Sometimes works as a hatch breaker when they are taking emergent midges. The side benefit is that you get to use a larger hook!
I also picked up a few fish on a midge pupa I tie, pumped the fish and found black fly adults and an olive midge larva, about a 24. Shortly after this I looked down and my leg was covered with adult black flies, switched to the adult imitation I posted earlier and it was on! Largest fish was about 16 inches, most averaged around 11-12. I had several fish break me off when I was swinging flies. I thought I saw a few mayflies riding the surface but didn't get a close look. They were ignored by the fish. My experience with black flies has shown that when the fish lock in on them they really don't want anything else. I also took fish on a Griffiths Gnat with a grey peacock herl body. I have not previously done well on this pattern when tied with regular peacock during this hatch..this inspired me to tie up a few with the grey peacock herl, grizzly hackle and a black head. I'll see how this works next time!
Hope this helps........