When you Blink you leave the normally visible area. Because the attacker doesn't see you coming when you blink back in, would you have advantage on the first attack roll?
No. It doesn't grant advantage on your attack, because you reappear at the start of your turn.
The description of the blink spell says:
Roll a d20 at the end of each of your turns for the duration of the spell. On a roll of 11 or higher, you vanish from your current plane of existence and appear in the Ethereal Plane [...] At the start of your next turn, and when the spell ends if you are on the Ethereal Plane, you return to an unoccupied space of your choice that you can see within 10 feet of the space you vanished from.
As you can see, the spell description states that if you do vanish from your current plane and appear in the Ethereal Plane, you return at the start of your next turn. (The spell's duration is "1 minute" and does not require concentration, so if you're still in combat when the spell ends, it still ends at the start of your turn, 10 rounds after you initially cast the spell.)
The rules on unseen attackers and targets state, in part (emphasis mine):
When a creature can't see you, you have advantage on attack rolls against it. If you are hidden — both unseen and unheard — when you make an attack, you give away your location when the attack hits or misses.
However, if you have the effect of blink active on you and you vanish into the Ethereal Plane at the end of one turn, you reappear in the Material Plane at the start of your next turn - which happens before you have even started to make any attacks at all. As such, you clearly don't have advantage simply by virtue of the blink spell.
The benefit of blink is not offensive, but defensive (in the sense that it allows you to potentially avoid danger). As the second paragraph of the spell description states:
While on the Ethereal Plane, you can see and hear the plane you originated from, which is cast in shades of gray, and you can't see anything there more than 60 feet away. You can only affect and be affected by other creatures on the Ethereal Plane. Creatures that aren't there can't perceive you or interact with you, unless they have the ability to do so.
The blink spell makes it so that, if you roll an 11 on higher on the d20 at the end of your turn, you simply can't be affected by most harmful attacks, spells, or other abilities. (Of course, your allies are still in harm's way - and there's also only a 50% that this happens, so you can't rely solely on blink to keep you out of trouble.) However, blink doesn't inherently grant any benefit to your attacks, because you reappear before those attacks happen - not as you're attacking, and not as a result of the attack.
This works with invisibility because the attack and the end of the effect of invisibility are synchronous - attacking and becoming visible happen at the same time, so you still benefit from the unseen attacker advantage.
With blink, there is nothing mechanically that guarantees the blink back and the attack happen at the same time - your target can see you when you blink back, and your attack is after you blink back. You are seen and do not have unseen attacker advantage.
A DM could rule otherwise.
This isn’t black and white. Much like the Ethereal Plane you blink to, this is shades of gray. It wouldn’t be unreasonable for a DM to give advantage on the first attack back from blink. But be aware, this does make the spell a bit more powerful than otherwise.
But it would not be overpowered.
Mechanically, it would still be inferior greater invisibility, a fourth level spell, which is right where a third level spell should be.