Generally associated with purple (Ex. 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 colour (Ex. 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 (Num. 4:6, 7, 9, 11, 12). (See Colour.)
Easton's Bible Dictionary. M.G. Easton. 1897.