Are underage non-nude sites and blogs legal?

It depends on where you are. Non nude is not the same as clothed in this situation. Most non nude sites are a way to get around child pornography laws. It has children/preteens/teens is compromising positions, and just clothed enough to get away with it some places. Many places are currently making moves to modify laws to make this illegal.

There is a reason that child pornography is illegal. It is abuse. Non nude is a technicality that skates on thin ice to get around it. You will have to check your local laws to see if it is legal in your area. However, legal does not always mean ethical.