Meaning

noun an annual Eurasian plant cultivated in North America having showy heads of blue or purple or pink or white flowers

noun blowfly with iridescent blue body; makes a loud buzzing noise in flight

originFrom Middle English blewbothel, from blew (“blue”) + bothel (“flower in the Asteraceae family”). In sense 2, a shortening of bluebottle fly. The fly was so named because it resembles the bluebottle flower in colour. The other transferred senses likewise have reference to the colour of the respective referents.

Etymology from Wiktionary, CC BY-SA 3.0