Albert Barnes Commentary 1 Kings 2:31

"And the king said unto him, Do as he hath said, and fall upon him, and bury him; that thou mayest take away the blood, which Joab shed without cause, from me and from my father`s house." — 1 Kings 2:31 (ASV)

Only a murderer was to receive no protection from the tabernacle. Hence, the reference to the “innocent blood.”