As for his sight he will remain blind through purgatory because purgatory “Is fire without its light.” The soul takes this in stride, "I am not worthy e'er to see again/The face of day". Despite being blind, Gerontius is told that he will see God for a split second during judgment.
Gerontius's blindness in purgatory is a consequence of his sins. He will remain blind throughout the process but will be granted a brief glimpse of God at the end of his journey.