One of my favorite books, “A Hog on Ice and Other Curious Expressions” by Charles Earle Funk, explains, “Anciently, a blush was a glimpse, a momentary view.” The expression evolved from that explanation of the expression to mean “…to redden as from embarrassment or shame.” I didn’t find an explanation why the expression evolved from meaning “a momentary view” to the current use  as described by Dictionary.com of “reddening from embarrassment.” Perhaps that is a mystery of the evolution of language.