John Calvin Commentary Psalms 69:23

"Let their eyes be darkened, so that they cannot see; And make their loins continually to shake." — Psalms 69:23 (ASV)

Let their eyes be darkened, that they may not see. The Psalmist here refers chiefly to two powers of the body, those of the eyes and of the loins; and I have no hesitation in considering his language as a prayer that God would deprive his enemies of reason and understanding, and at the same time enfeeble their strength, so that they might be altogether unsuited for exerting themselves in any way.

We know how indispensable it is, if anything is to be done rightly, that counsel should first provide illumination, and that the power to execute what is planned should also be added.

The curse expressed here impends over the heads of all the enemies of the Church; therefore, we have no reason to be terrified by the malice or fury of the wicked. God, whenever he pleases, can strike them suddenly with blindness, so that they see nothing, and by breaking their loins, lay them prostrate in shame and confusion.