Where did the expression “at first blush” come from and What does it mean?

Anciently, a blush was a glimpse, a momentary view.

This was the sense that we find in the late fourteenth-century poem, “Joseph of Arithmathie”: “Aftur the furste blusch we ne michte him biholden (After the first glimpse we could not behold him).”

This sense dropped out of use during the sixteenth century, however, except in the present phrase.