They aren't ripe for picking until end of July/mid August. You will need permission of the landowner to pick unless you are sure it is crown land, not Indian land, provincial or national park land or protected land.

Usually found on dry, rocky, granite based soils in full sun. Most blueberries in Ontario will be low bush. Rocky pink granite outcrop areas near lake or rivershore with juniper growing nearby would be good places to look. If you are driving near forested areas, much less likely to find them, they don't grow in good, deep soil nor in forests.

You are near the French River; have you ever taken out a canoe on any of the canoe routes? Should be blueberries somewhere along them.