When spectating a living entity, the player does not truly "see the world through their eyes"; any vision-impairing effects the mob has (Blindness/Darkness/Nausea/wearing a carved pumpkin) will not be visible. Instead, any visual effects on the player will continue to be visible, with the exception of the fog from Blindness/Darkness, which will be replaced by the ambient fog. (The gamma reduction from Darkness pulses on the player is still visible.)

Expected behavior

When spectating a living entity, the player would be able to see any applicable Blindness/Darkness/Nausea/pumpkin overlay effects for that entity, while any such visual ailments the spectator themselves would normally experience would not render.

P.S.

If this issue is fixed for the above cases, and it is decided (surely wisely) to not make the screen completely black when spectating a warden, a Mojang comment about the reasoning would be appreciated.