Negan is employing a figure of speech. He is saying, presumably in answer to a question, that his mind never becomes empty, that, in fact, when he has nothing to think about, his mind turns to a topic that would make the other person blush so much that his collar would fly off his shirt. This is a figure of speech called hyperbole, (an exaggerated statement or claim not meant to be taken literally). It seems to be made up by the scriptwriters for Negan to say. The transitive use of 'blush' is non-standard. The figure of speech is similar to established idioms where strong feelings or emotions are imagined to produce actual surprising physical effects, e.g. cry one's eyes (or heart) out, sob one's socks off, etc. In comics and cartoons it can be a convention that extreme surprise makes a person's hat fly vertically from their head. The topic to which his mind turns in probably sex, as is suggested by the mention of the other person and "Rosita" being "a thing" (in a relationship).