There was a day when sunroofs were opaque, either fabric that folded back (think: VW Bug) or solid metal sliders (think: Mercedes.) I could be wrong, but I believe the term 'Moonroof' originally refered to tinted glass, diminishing the sun's effect and turning it a bluish color, which is now a much more ubiquitous style than the solid 'sunroof.'

Or I could be completely wrong.