Why does Black Gram Batter turn fluffy only when stone ground?
A blender will cut or shatter what you put into it while leaving the structure of the pieces relatively intact. A grinder or mortar will apply pressure, causing cell walls to rupture, so molecules in the food that were previously kept separate can react, and fermentation relevant microbes (which, if I remember correctly, are important for idly?) have good access to all of the lentil and not just ruptured cells at the edges...