When Milton asks, "Doth God exact day-labour, light denied?" he seems to be using a term common in his day. "Day-labour" would apparently mean common outdoor labor performed from sun-up to sundown. A person who performed this kind of work would have been called a "day-labourer," and that term may still be heard in some places in America today. What Milton seems to be asking, rhetorically, and half-jokingly, or fondly, or foolishly, is, "How can God ask me to work in the daytime and then not give me any daylight?" If he is making a joke, it is a grim joke--which is probably why he says "fondly" rather than "jokingly."