Easton’s Bible Dictionary

Blue : 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 colour (Exodus 28:31), also the loops of the curtains (Exodus 26:4) and the ribbon of the breastplate (Exodus 28:28). Blue cloths were also made for various sacred purposes (Numbers 4:6,7,9,11,12). (See COLOUR.)