Rashi on Exodus 28:31/רש"י על שמות כ״ח:ל״א

1את מעיל האפוד THE ROBE OF THE EPHOD — i. e. the robe over which the ephod was placed to serve the purpose of a girdle.

2כליל תכלת signifies THE WHOLE OF IT BLUE PURPLE, meaning that there should be no other material mixed with it (Yoma 71b).