Susanoo is unlocked when the user has two MS abilities, one in each eye resulting in the third final power being unlocked Susanoo. Susanoo once unlocked is actually not a sharingan power anymore(No eyes needed), its literally the users will manifest. This was proved when eyeless madara used susanoo. Think of it like this, when the two MS powers/chakra mix in the brain it results in the third power(Susanoo) being unlocked, however unlike the other two only the users will is required to manifest this ability. This being said i believe eyes are required to form/stabilize perfect susanoo, i could be wrong however this is my opinion. 
As a Naruto fan, i shouldn't have to school you uchiha faps. :bdpf:
As a Naruto fan, i shouldn't have to school you uchiha faps. :bdpf: