1A greyish-blue colour.
A small dark mayfly of the genus Baetis; an artificial fly made in imitation of it.
3Any of various blue pigments containing iron, especially. Prussian blue.
In earlier apparent examples, following the verb precipitate, blue is used (as are other colour adjectives) as predicative complement to iron.
Of a greyish-blue colour; designating this colour.
Recorded earliest in "iron-blue fly".
Late 17th century; earliest use found in John Dryden (1631–1700), poet, playwright, and critic. From iron + blue.