Because there are about forty species, not all flourish at the same time. Black flies can travel several miles from their breeding site, so those environmental rules cannot be counted on completely, as a means to avoid them. Strong breezes tend to disperse them, as they are a very small insect. I have found, and others confirm, that black flies are generally inactive until the air temperature has risen to at least 50 degrees F., even in black fly season. In general, unlike mosquitoes, they will not come indoors, nor fly at night, nor penetrate most clothing.
Local variation is the rule.
Local people are usually the most helpful resource, as they observe these things very carefully. There are many species in North America. They all breed in running water. The females bite to get a meal of blood. They are a nuisance to humans in Northern areas from approximately May through July.