Can you use your reaction while ethereal from the blink spell?

Question

If you have blink active and roll over 11 at the end of your turn, you're no longer on material plane. Are you then able to use your reaction?

E.g. During turn, X melee attacks Y then ends turn in ethereal plane. Y moves during their turn lower in initiative order and would normally trigger an AoO, but X, being in ethereal can't take the attack.

Additionally, if X has pole arm feat and ends turn in ethereal realm only to have Y appear 10ft away from them in ethereal realm later in turn, X can take AoO against Y there ?

Answer 1

Yes. Nothing in the Blink spell description prevents you from using a reaction that you have.

However, there are some common reactions that no longer apply or may be less useful when you're on the ethereal plane. For example, when on the ethereal plane you can't attack someone on the material plane.

"You can only affect and be affected by other creatures on the Ethereal Plane." - PHB p219

"Normally, creatures in the Border Ethereal can't attack creatures on the overlapped plane, and vice versa" - DMG p48

This means an attack of opportunity reaction will only be useful if something else on the ethereal plane triggers that reaction.

If you have another reaction, such as a readied action to attack if an enemy enters the ethereal plane within striking distance, that will trigger normally.

Answer 2

Nowhere in Blink's description, or the description of the Ethereal plane on page 48 of the DMG, does it say anything about limiting your use of reactions in any way. So yes, you would still be able to use your reaction while on the Ethereal plane.

You can't, however, use your reaction to make an opportunity attack against anyone on the plane you originated from because "you can only affect and be affected by other creatures on the Ethereal Plane" and they are not a creature on the Ethereal plane.

As for Polearm Master, I'd say it would depend on the manner in which the creature enters the Ethereal plane. Creatures don't provoke an opportunity attack when they teleport or when someone or something moves them without using their movement, action, or reaction.

Blink for instance could be thought of as a form of teleportation, but it also doesn't use the creature's movement, action, or reaction to get to the Ethereal plane so it might not provoke an opportunity attack.

Whereas a Ghost's Etherealness ability does use it's action to enter the Ethereal plane so that might provoke an opportunity attack.