Psalms 135:21 - Matthew Poole's English Annotations on the Holy Bible

Blessed be the Lord out of Zion; by the assemblies of his people in Zion or Jerusalem. Which dwelleth at Jerusalem: this clause may be added either to distinguish the true God from the gods which were worshipped in other places and countries; or as a reason why they should bless God, because he had blessed and honoured that place with his gracious and glorious presence.