The phrase "beat black and blue" is a violent idiom used to describe the act of physically assaulting someone. If you are looking for synonyms to this phrase, you could use terms such as "thrash," "pummel," "assault," "attack," "maul," "batter," "bludgeon," "hit," "strike," or "wallop." Each of these words conveys a sense of physical violence and aggression that is similar in meaning to the phrase "beat black and blue." Using these synonyms can help add variety to your writing and make your descriptions more vivid and engaging.