Optical effects are several generations removed from the original so that seems like a factor.
Looking at Dark Jedi's LD conversion, this dark stuff is static so I don't think it's grain, I think it's that the Tatooine shots used nylon hose (leggings) over the lens to give it a
diffuse, otherworldly look.
However, Harmy's 720P version doesn't have it, so it may have been cleaned up/lightened for the later hi-def/SE editions?