His watchmen are blind. Israel's "watchmen" are his guides and teachers, the prophets (Isaiah 6:1-13:17; Ezekiel 3:17; Habakkuk 2:1, etc.). At the time of which Isaiah speaks, they are "blind" (Isaiah 29:18; Isaiah 35:5; Isaiah 42:7, Isaiah 42:16, Isaiah 42:18, Isaiah 42:19; Isaiah 43:8, etc.
), or without knowledge—like the "blind guides" of the Gospel (Matthew 15:14; Luke 6:39, etc.). They have not the spiritual discernment which would enable them to lead the people aright. Further, they are dumb dogs.
Instead of acting as faithful watch-dogs, who give warning of the approach of danger by their barking, they remain apathetic, and utter no warning at all. It is as if they passed their lives in sleep.