There is a proverb "Love is blind", meaning that we don't see the faults in the one we love (even if everybody else can). "Blow is blind" is modeled on that. "Blow" is slang for cocaine. Adam's father has corrupted the proverb by changing the meaning of "blind" into a more literal meaning. When you are high on cocaine, you can't see it when a woman is ugly. I can only hope that this sordid saying is part of the development of the character of Adam's father as a low-life creep.