Black or white thinking is a thought process that occurs when we go from one extreme to another. We fail to see the middle, or grey area, rather we bounce from one extreme (white) to the other (black). This way of thinking is actually a defense mechanism, designed to protect us from discomfort.
Maybe we view people as fitting into two categories, a “good” guy or a “bad” guy. If a person does something that's not within the good guy persona we've created, they become a "bad"