Blue Streak is an idiom that refers to someone who talks incessantly and rapidly. Some of the antonyms for blue streak could be calm, quiet, or soft-spoken. Another option could be the word "reserved," indicating someone who is more introspective and less likely to talk at length without prompting. "Measured" and "deliberate" also present antonyms to blue streak, depicting someone who takes care and time to express themselves thoughtfully. Finally, "reticent" describes someone who is shy or guarded and tends to speak infrequently, the opposite of someone who goes on a blue streak.