At the first blush. At the first glance; speaking off-hand without having given the subject mature deliberation. The allusion is to blushing at some sudden or unexpected allusion; the first time the thought has flashed into your mind.
“England might blush in 1620, when Englishmen trembled at a fool's frown [i.e. James I.], but not in 1649, when an enraged people cut off his son's [Charles I.] head.” —Wendell Phillips: Orations, p. 419.