How did josh greene of promised Land become blind?

During a crossover episode with Touched By An Angel, Tess called on him to help Monica who had become temporarily blind so that he could help her on her assignment and learn to become a man and see the world not with his eyes, but his heart. While helping Monica, he was assisting her "assignment" clean graffiti off a wall when the gangs that tagged the wall committed a drive-by shooting. He was shot and also obtained a head injury which is what, I guess, caused his blindness.