Do people ever get the undirected urge to blog but don't know what to blog about? Usually people blog about the things they care about, and don't need suggestions.
Most bloggers i know, write because they are genuinely interested in an activity/thought/event. If someone writes about something they are not really interested in (or knowledgeable about), it would be plainly visible. (One of the exceptions i can think of are people who write pure fiction like short stories on their blog ala writers.)