This question essentially boils down to defining the distance between planes and that matter has been addressed already in this question.
To wit, there is no standard definition of interplanar distance in the game, though certain spells address the matter on a case-by-case basis for their own purposes.
While Locate Object is magical, which would seem to indicate that it satisfies Etherealness' clause stating "unless a Special ability or magic has given them the ability to do so," the fact is that Locate Object does not expressly grant detection across/between planes.
Therefore, because spells only do what they say they do (and nothing more nor less), we have to conclude that Locate Object does not grant its caster the ability to detect across planes.
The way I interpret true seeing is that if you can see an area, then you can see any ethereal creatures or objects in that area (among other things). If you're in a fog cloud, or wearing a blindfold, or have a bag over your head, you can't see at all, so your true seeing is functionally blocked as well.
As far as walls go, we're told that material objects seen from the ethereal side are "muted and indistinct", "shadowy", and that they don't "hamper movement", but nothing says you can see through walls simply by being on the ethereal plane. I see no reason truesight would be different.
The DMG doesn't go into a ton of detail about other planes, but one of my favorite 3rd Edition resources was the Manual of the Planes, which was very specific about this:
To viewers on the Ethereal Plane, objects on the Material Plane are foggy, indistinct, and almost translucent. Such objects block line of sight and provide concealment, but not cover. An ethereal observer can't see through a wall on the Material Plane unless he pokes his head through it first. An ethereal character whose eyes are totally within a Material Plane object cannot see.
That doesn't exactly address the question you're asking, but I think it's clear that the intent is that your field of vision doesn't change when you're seeing into the other side, only what you can see within that area.
Best Answer
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:
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:
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:
Beyond that, you use your own sight. That means all other restrictions of your own sight are still there:
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):