Usually, when two creatures can't see each other, and one makes an attack roll against the other, we roll a straight single D20-- advantage from unseen attacker cancels out with disadvantage from unseen target.
An echo knight's echo, however, is not a creature, but an object (as typically interpreted, with guidance from Crawford: "If this translucent, gray image were meant to be a creature, the rule would say so.") Perhaps redundantly with that, it is also immune to all conditions (I'm not sure that anything that is not a creature can even have conditions), so it can't be Blind, as heavy obscurement or attacker invisibility would impose.
So can attacks against an echo even gain advantage from unseen attacker?
If they can, can attacks against other objects gain advantage from unseen attacker?