The colour of the far sky and deep water. Also low in mood — one of the oldest crossings between sight and feeling.
Old French bleu, of Frankish origin, from Proto-Germanic *blewaz; it displaced or merged with a native Old English word for the same colour, and the low-mood sense is recorded from the 17th century.
The colour of the far sky and deep water, carried into feeling as one of the oldest crossings between what is seen and what is felt.