Does Blinking creature loss the benefit on attacking if the opponent can see invisible creature?

Question

You strike as an invisible creature (with a +2 bonus on attack rolls), denying your target any Dexterity bonus to AC. (Blink)

If the attacker can see invisible creatures, the miss chance is also only 20%. (For an attacker who can both see and strike ethereal creatures, there is no miss chance.) Likewise, your own attacks have a 20% miss chance, since you sometimes go ethereal just as you are about to strike. (Also Blink)

So, if the opponent wants to completely negate the defensive benefit of Blinking, that opponent needs to both be able to see invisible creatures and be able to hit ethereal creatures.

Do they negate the offensive benefit by simply being able to see invisible creatures? Or do they also need to be able to hit one?

Answer 1

Yes

There is no offensive benefit to being ethereal. In fact, there is an offensive penalty, which is represented by your own 20% miss chance. This is because, as a creature in the Ethereal Plane, you can't damage a creature that is in the Material Plane with most effects (maybe there are a couple of exceptions, but even Force effects do not extend into the Material Plane if cast from the Ethereal).

So the only offensive benefit to blinking is that fact that it's hard for an enemy to follow your movements, because you're inivisble half the time. If the foe can see creatures in the Ethereal Plane, this benefit is utterly lost.

TL;DR: the offensive benefit of blink's effect comes from the fact that you're ethereal (and thus invisible) only some of the time. If you were ethereal all the time, you would not be able to harm creatures in the Material Plane at all. If the enemy can see into the Etheral Plane, you aren't invisible at any point. Thus you have no offensive benefit.