Charles Spurgeon Commentary Numbers 4:5-6

"when the camp setteth forward, Aaron shall go in, and his sons, and they shall take down the veil of the screen, and cover the ark of the testimony with it, and shall put thereon a covering of sealskin, and shall spread over it a cloth all of blue, and shall put in the staves thereof." — Numbers 4:5-6 (ASV)

These Kohathites were not permitted to handle the ark directly when they transported it; much less were they ever to look at it. But the priests, the sons of Aaron, went in first, and after carefully covering the holy furnishings, they covered the sacred ark with a cloth of blue. Blue was the symbol of holiness—of separation. Therefore, every Israelite wore a border of blue on his garment; but this, which was the symbol of the divine presence, was all of blue. It is all holiness. We wear, alas!, only a border of blue; but this holy thing was all of blue.