Let's say I attack with Aetherling. After it deals damage I blink it out. When it returns at my end step, does it come back untapped and ready to block?
Yes, when Aetherling returns to the battlefield, it's considered a new permanent, so it enters the battlefield without remembering that it was tapped.
400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are seven exceptions to this rule:
(none of the exceptions apply here).