Isaiah sets up a contrasting set with verses 10 and 11. The image is darkness versus light. As with most of the darkness/light images, the context is one in which an individual will either be in darkness or light. Also as usual, light is connected with obedience to the gospel.
The King James language of these verses is not as clear as it might be. Gileadi's translation of verse 10 is:
The servant is setting up Israel as hypocrites. That while they profess to follow the Lord, yet they do not. Thus the servant addresses one who might proclaim to fear the Lord, yet walks in darkness.