Maybe I'm crazy, but I've never had the blackflies be so bad in May that they've ruined a trip for me. They're annoying, but nothing that can't be combated relatively easily. I've always found June to be the worst in the northeast, but still... they've never stopped me from going outside entirely.
I wouldn't let the naysayers talk you out of a trip you really want to do, just because some bugs will eat you. That said, I'll probably go for a walk next month and be eaten alive.