Let me lay out the scenario first: a human in the dark enters a pitch-dark room. Within the room is an elf who watches the human enter. The elf is not trying to hide, just standing there. They are enemies. The human wins initiative.
Now, per the rules the human has the blinded condition, which gives disadvantage on attack rolls. However, the elf isn't hiding, so with a perception check the human should be able to locate him, cross the room and attack, with disadvantage.
Is this correct? Does he have disadvantage on the perception check? Perception is not just sight and since the elf isn't trying to hide then I'd say no, it's just a straight perception check. And finally, since the elf isn't hiding, what would the DC be for such a check? Arbitrarily assigned by the DM?
There are other factors that I won't get into (such as: if the human is a rogue, he should be able to hide from the elf with stealth, since darkvision turns darkness into dim light; or if he were a barbarian, he could negate the disadvantage from blindness by reckless attacking and cancelling it out with advantage).