The description of the Ghost says:
Ethereal Sight. The ghost can see 60 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
It is visible on the Material Plane while it is in the Border Ethereal, and vice versa.
Could that mean the Ghost can see through or rather on the other side of a darkness spell on the Material Plane?
Is Ethereal Sight described somewhere else?
I have found the following in DMG p. 48:
From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. [...]
Which does not answer the question of: Can we see on the other side of a darkness spell (or even through it)?