If an attacking creature is blinked after blockers are declared does it still deal damage or leave combat?

When a creature is "blinked" (exiled and returned to the battlefield), it becomes a completely new game object with no connection to its previous existence. This is governed by CR 400.7, which states that an object that moves zones is treated as a new object unless a rule specifically says otherwise.

Because the returning creature is a new object, it has never been declared as an attacker in the current combat. It is therefore not in the 'attacking' zone (the 'attack' designation is tied to the original object), and CR 506.4 confirms that a creature must be declared or put into combat to be considered attacking. The new object simply enters the battlefield under its owner's control — it is not in combat at all.

Since the creature is no longer an attacking creature, it will not deal combat damage during the Damage Step (CR 510.1) and any blockers that were assigned to it become unblocked creatures with no attacker to damage. Those blockers simply remain blocked for the rest of combat but deal no damage to the original attacker.

Example: You attack with a 3/3 Grizzly Bears. Your opponent blocks with a 2/2. In response to nothing (or to a spell), you blink the Bears with Ephemerate. The Bears return as a new object — they are not attacking, not blocked, and will not deal or receive combat damage. The opponent's 2/2 remains 'blocked' status but has nothing to deal damage to.

Note: The blocker's 'blocked' status does not go away (CR 509.1h), so the blocked creature cannot deal damage to the defending player either, unless it has trample or a similar ability. Only the blinked creature's combat participation ends entirely.