An Etymological Dictionary of the German Language/Annotated/Blick
Blick, masculine, ‘glance, look, gleam,’ from Middle High German blick, ‘splendour, lightning, glance’; corresponds to Old High German blic (blicches), neuter, ‘lightning’ (also blicfiur, ‘electricity’). The original sense of the Middle High German word was probably heller Strahl (a bright flash), Strahl being used figuratively of the eye as of lightning; the physical meaning of the stem has been preserved in Blitz. The root is shown under blecken, and especially under Blitz, to be the pre-Teutonic bhleg.