Albert Barnes Commentary Psalms 129:8

"Neither do they that go by say, The blessing of Jehovah be upon you; We bless you in the name of Jehovah." — Psalms 129:8 (ASV)

Neither do they who go by say, The blessing of the Lord,... - This describes a scene in a harvest-field, where persons passing by express their joy and gratitude that their neighbors are reaping an abundant harvest.

The phrase The blessing of the Lord be upon you, was expressive of good wishes, of pious congratulation, and of a hope for success and prosperity, much as when we say, “God be with you,” or, “God bless you.”

The meaning here is that such language would never be used in reference to the grass or grain growing on the housetop, since it would never justify a wish of that kind. It would be ridiculous and absurd to apply such language to anyone who might be found gathering up that dry, withered, and worthless grass.

So the psalmist prays that this may be the case for all who hate Zion (Psalms 129:5): that they may have no such prosperity as would be represented by a growth of luxuriant and abundant grain; no such prosperity as would be denoted by the reaper and the binder of sheaves gathering in such a harvest; and no such prosperity as would be indicated by the cheerful greeting and congratulation of neighbors who express their gratification and joy at the rich and abundant harvest that has crowned the labors of their friend, by the prayer that God would bless him.

We bless you in the name of the Lord - This is still the language of pious joy and gratification addressed by his neighbors to him who was reaping his harvest. All this is simply language drawn from common life, uttering a prayer that the enemies of Zion might be confounded and turned back (Psalms 129:5); a prayer that they might not be successful in their endeavors to destroy the Church. Such a prayer cannot but be regarded as proper and right.