Centaurea cyanus, the European cornflower is an aster which once grew as a weed across Europe (particularly in grain fields). As agriculture has grown more sophisticated (and herbicides more puissant), the cornflower has become uncommon to the point of extinction in its native habitat. Yet the cornflower is far from gone: its bright blue color means that some enthusiasts grow it as an ornamental garden plant. Additionally, in the era before herbicides and intensive agriculture, cornflower seeds frequently contaminated planting seeds—which meant that the cornflower traveled to Australia, the Americas, and Asia where it quickly became invasive.
The cornflower, also known as the bachelor button or knapweed is the national flower of Germany. It has long been traditional for unmarried men to wear one in their buttonhole (although I abjure this practice myself).
The most famous aspect of cornflowers is their dazzling bright blue color which inclines very slightly towards purple. For centuries, this color has been a favorite of tailors, decorators, dressmakers, and artists. Cornflower blue is thus a classic traditional name for this brilliant midtone blue: indeed the color was very much a favorite of Vermeer. The name is still very much in use, so it is perfectly correct to imagine some charlatan or fop of the Restoration era donning a cornflower coat of the same color as the bridesmaids will be wearing at your cousins’ wedding next week.