I have been getting into some very small biting flies during wet years. They are not black flies but may have related habitat requirements. Currently they are concentrated near ponds but since we are having a major flood event they will likely be found further afield.Actually are probably Black Flies.
Were they in the coop too?
Haven't had them that bad, but have read a fan and/or real vanilla will repel them from the coop/roost area.
Sorry for your losses.