It is entirely possible for your critique of “Blue Lives Matter” to be spot on and for the the other person’s choice to wear the clothing bearing that message to be protected by the First Amendment at the same time. Indeed, his right to do so gave you the opportunity to explain the problem with his thinking on the subject. Driving stupid ideas underground won’t stop people from having them. However, it will prevent us from having a chance to confront them and potentially persuade them to change their mind.
Good on you for sharing your thoughts with him but don’t forget that the only reason you knew he had the point of view he did is because he could wear that clothing. All bans on certain types of expression do is protect us from seeing or hearing opinions we disagree with. That produces a false sense of reality/security that can be far more dangerous in the long-term than the expression itself. Just because we don’t see or hear actually/potentially wrong ideas it doesn’t follow people don’t still hold them. Sunlight is the best disinfectant. Freedom of speech is intended to facilitate (eventually) getting at the truth, not hide from it.