Albert Barnes Commentary Numbers 4:6

"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 4:6 (ASV)

wholly of blue - Compare Exodus 25:4 (see note). The third and external covering of the ark only was to be of this color. The table of showbread had an outer wrapping of scarlet (Numbers 4:8); the altar, one of purple (Numbers 4:13).

Put in the staves - Probably, “put the staves in order.” These were never taken out of the golden rings by which the ark was to be borne , but would need adjustment.