וְנָתְנ֣וּ עָלָ֗יו כְּסוּי֙ ע֣וֹר תַּ֔חַשׁ וּפָרְשׂ֧וּ בֶֽגֶד־כְּלִ֛יל תְּכֵ֖לֶת מִלְמָ֑עְלָה וְשָׂמ֖וּ בַּדָּֽיו׃
and shall put thereon a covering of sealskin, and shall spread over it a cloth all of blue, and shall set the staves thereof.
Read kabbalistic (mystical) interpretation of Numbers 4:6: and classical mystical sources.