Albert Barnes Commentary 2 Kings 6:18

"And when they came down to him, Elisha prayed unto Jehovah, and said, Smite this people, I pray thee, with blindness. And he smote them with blindness according to the word of Elisha." — 2 Kings 6:18 (ASV)

They came down to him: The Syrians, who had been encamped on the high ground opposite the hill of Dothan, now descended and drew near to the city.

The blindness that struck them was not a literal loss of sight, but rather a state of illusion in which a person sees things differently than they really are (Compare to 2 Kings 6:20).