Darkvision enables you to see in darkness as if it were dim light (within the first 60/120/180 feet, of course, depending on the quality of your darkvision).
The Eagle totem for the 6th-level Totem Barbarian's Aspect of the Beast feature enables you to ignore your disadvantage from dim light on sight-based Perception checks.
Does that mean that a character with both darkvision and the eagle aspect can make Perception checks (within his darkvision range) without disadvantage, even when in complete darkness ?
Best Answer
It's a rather nested definition, but...
Out to the range of their Darkvision, yes.
Darkvision says (emphasis added):
Dim light says (more emphasis added):
Light obscurement is:
A character with Aspect of the Beast: Eagle (PHB, p. 50) or the Skulker feat (PHB, p. 170) doesn't take disadvantage for that dim light.
Summary
A character with Aspect of the Beast: Eagle can see a mile in dim light and does not suffer disadvantage to Perception due to that dim light.
A character with Aspect of the Beast: Eagle (or Skulker) and Darkvision can see the full range of their darkvision and does not suffer disadvantage to Perception due to that dim light. Beyond that range, they're Blinded like any other character.
Caveats
There are other things that can cause obscurement - fog and foliage, for instance. Neither Aspect of the Beast: Eagle nor Skulker help with that, only with the obscurement caused by dim light.