Embarrassment, awkwardness, or being caught off guard — the blush strongly signals a flustered, self-conscious reaction, often to a compliment, an accidental slip, or something mildly scandalous. It's also widely used, somewhat euphemistically online, to imply flustered attraction or a suggestive reaction.
Keywords: amazed, awkward, crazy, dazed, dead, disbelief, embarrassed, face, flushed, geez, heat, hot, impressed, jeez, what, wow