I've fished all over Quebec for years. The black flies are generally at their worse through June and then dwindle gradually. We mainly fish for Walleye and some Pike Fishing midday. We have had tremendous luck most years but every once in a while its just not a good fishing week.
In July the walleye are a little less concentrated and they start to suspend in deeper waters. Once you find them its fine, but it might take a day or two. Be sure to ask the guys who are checking out when you get there. Not always reliable but it can be helpful. One time we asked two guys who were leaving an outpost how the fishing was. They replied the worse fishing they had ever seen, they doubted there was any fish in the entire lake.
We caught over 100 walleye that very night (returned most of them to the drink).
Wildbill51