Black fly season is hard to nail down. Temperatures play a big role. If you hit it at the peak it can quickly turn a good time to miserable. Some of the local stores shut down during Black fly season because they know that people aren't heading out to the woods. If you do go, get a no see-um suit, lots of bug spray, cigars and don't use Thermacell products. They attract Black flies.
Maybe try for a little bit into June. Black flies are usually done and temps aren't as bad as July and Aug.