This color is generally associated with purple (Exodus 25:4; 26:1, 31, 36, etc.).
It is supposed to have been obtained from a shellfish of the Mediterranean, the Helix ianthina of Linnaeus.
The robe of the high priest’s ephod was to be all of this color (Exodus 28:31), also the loops of the curtains (26:4) and the ribbon of the breastplate (28:28).
Blue cloths were, also, made for various sacred purposes (Numbers 4:6-7, 9, 11-12).