The ongoing damages aren't attacks, and are unaffected by mirror image

Quoting the text of mirror image with emphasis:

Each time a creature targets you with an attack during the spell's duration, [...]

"Attack" has a specific definition (see this answer for more detail); it's only an attack if it requires an attack roll. Once the initial attack hits, subsequent applications of damage no longer require an attack roll, therefore they're not attacks as far as the rules are concerned. Since Mirror Image only works on attacks, it is ignored.

It depends on the caster's sight

Appendix C of the Player's Handbook (or the basic rules) says under the "Transitive Planes" heading that some spells allow you to see into the Ethereal Plane:

Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Certain creatures can see into the Border Ethereal, and the see invisibility and true seeing spell grant that ability.

The see invisibility spell allows you to see into the Ethereal Plane. But any other ability to see into the Ethereal Plane (or vice versa), does NOT give you all the effects of the see invisibility spell.

The see invisibility spell description explicitly says it gives you two different abilities — to see invisible things AND see into the Ethereal Plane:

For the duration, you see invisible creatures and objects as if they were visible, and you can see into the Ethereal Plane.

Since spells do only what they say they do, the Blink spell gives you an ability to see the Material Plane from the Ethereal Plane, nothing more. It does NOT give you the other effect of the see invisibility spell.

So you simply can see all the Material Plane — terrain, objects (including walls) and creatures. The blink ability imposes two restrictions though:

• You can't see farther than 60 feet into the Material Plane
• You can't see colors when perceiving the Material Plane

Beyond that, you use your own sight. That means all other restrictions of your own sight are still there:

• If you can't see through fog normally, you can't see through it from the Ethereal Plane
• If you can't see through darkness, you can't see through it from the Ethereal Plane
• If you can't see through walls, you won't see through walls from the Ethereal Plane
• If you can't see invisible creatures, you won't see them from the Ethereal Plane

The blink spell does not give you an ability to see invisible creatures, or see through walls. You will neither see through walls, nor invisible things, unless you already were able do that before casting blink.

Also, you will see the Ethereal Plane itself

The blink spell makes you "vanish from your current plane of existence and appear in the Ethereal Plane". It also says that you "can see and hear the plane you originated from". Since it doesn't say "can see only the plane you originated from", I assume you can see the Ethereal Plane as well, including creatures from this plane.

However, you can't look farther than 60 feet in Ethereal Plane (see DMG page 48, under the "Ethereal Plane" heading):

Visibility in the Border Ethereal is limited to 60 feet