c. 1200, from Old English blostm, blostma "a flower of a plant," from Proto-Germanic *blo-s- (source also of Middle Low German blosom, Dutch bloesem, German Blust), from PIE *bhlow-, extended form of root *bhel- (3) "to thrive, bloom." This is the native word, now largely superseded by bloom (n.1) from Old Norse and flower (n.) from French.
late 14c., from Old English blostmian "put forth blossoms, to flower," from blostma "a blossom, a flower" (see blossom (n.)). Figurative use from late 14c. Related: Blossomed; blossoming.