Simple question, I think. I'm referring to an environment where each PC has total concealment from one another.
Depends. Does the aid rely on visual cues?
I'm sure Aid Another is doable for a Perform (oratory) check or something like that; if a blind person could help without touching you or your work, then concealment is irrelevant.
To make an Aid Another check to increase an ally's AC or attack roll, all it requires (by RAW as far as I can tell) is a melee attack roll against a foe both you and your ally are fighting.
The rules then state that 'you can also use this standard action to help a friend in other ways,' but honestly I don't believe a Perform skill check is ever mentioned unlike the 3.5 counterpart.
From our Commander class:
Direct Order (Ex): A commander learns how to instruct his allies in the heat of battle, shouting orders over the chaos that ensues. At 1st level, as a standard action, he can issue a direct order to one ally within the radius of his commanding presence. Giving direct orders allows the commander to aid another in combat using the Perform (oratory) skill check (DC 10 or higher) rather than making an attack roll against AC 10. Allies must be able to hear the commander to gain the benefit.
I'm sure you can make a house rule for it though.
I'd suggest this is situational and depends on the type of aid and the role-playing. For example, if my intention is to aid someone else in a Stealth check by providing a distraction, I don't need to see or hear them.
If my intention is to aid someone else in a Perform check by acting as a shill in the audience, I would have be watching the performance so I knew how to clap.
Shouting "there's a trip wire on the floor!" can help someone's Perception check whether I can see them or not. But if I'm trying to block a blow aimed at an ally's head, it would probably help to see the head.