What is the Shadow Elrond refers to?
From FOTR:
"I can foresee very little of your road; and how your task is to be achieved I do not know.   The Shadow has crept now to the feet of the Mountains, and draws nigh even to the borders of the Greyflood;  and under the Shadow all is dark to me." 

What Shadow is Elrond referring to here and how is it that it has such a distinct border?    

It can't be a metaphorical reference to the Black Riders, since they have already penetrated far west of the Greyflood, over the Brandywine and into the Shire, and Elrond knows it well. 

It can't be the literal sunless cloud that Pippin witnesses in Minas Tirith, for that comes much later in the tale. 

It might be read as a metaphorical reference to the Wargs coming west of the Mountains, or to the crebain from Fangorn and Dunland.    If it is a metaphorical reference to a real world danger, how is it that none of the Company take it seriously enough to take warning from it?

Taking that statement to indicate something to do with the power of the ring Vilya, it's a risky statement for Elrond to make for no apparent benefit to the Company.
The Shadow clearly refers to the growing influence of Sauron. I think you can interpret the overall sentence in one of two ways.

1. Elrond has some literal power of far sight, whether inherent to him or derived from Vilya, and that power of far sight is blocked by Sauron’s influence.

2. Because of Sauron’s growing influence and power, Elrond’s more prosaic sources of intelligence such as messengers and traveling Eldar are less reliable.
Always after a defeat and a respite, the Shadow takes another shape and grows again.

I don't have any humble opinions.
I have trouble with the first reading because Sauron's influence under that interpretation seems weirdly geographically bounded, and we have no stated mystical mechanism (e.g. palantir, Amon Hen) that would have such specific bounds.    I will acknowledge that Sauron cannot see into Lorien, but that is specifically due to Galadriel's power and not due to pure geography.     There is no one of similar power to contest Sauron at the Greyflood.      

As I explained above, I really don't want to use the first reading in a Vilya context, because then Elrond making such a statement would be a severe breach of  Three Ring security, to no particular benefit.    Further, Galadriel with weaker Nenya can see into Sauron's devisings whereas Elrond somehow cannot with Vilya?

The second reading seems a bit forced to me, as prosaic messengers and traveling Eldar do not really relate to foresight.    If we ignore that for the sake of discussion, then Elrond is being a bit redundant both to his later specific statement about messengers and to Aragorn's knowledge (derived from the Dunedain and from Elladan and Elrohir's debriefing).

Given a choice between the three readings and no other, I will pick 1A and infer a geographically bounded limiting mechanism to mystical power.    I am still uncomfortable importing my inventions into the main line of the story.   (Insert undecided emoji here)
It seems to me that you’re reading too literally and inferring a sharp binary from the clause “under the shadow all is dark to me.” What’s Elrond really saying here? Nothing more or less than that he doesn’t have good information on conditions east of the Greyflood or the Misty Mountains, so he doesn’t have much useful advice to give the Company. Whether he gets his information via mystic or prosaic means is really secondary to the fact that he doesn’t have any.

I’m inclined to think that, while we cannot fully rule out some mystic means as a supplemental source of information, Elrond’s primary information source is prosaic. Why else delay the quest for two months (into the dead of winter) while scouts and messengers are sent out? Since we also know that no scouts went further afield than Lorien, the “Shadow” comment merely acknowledges what should be obvious...we just don’t know what conditions are further east and south, because the shadow holds sway and no reports are coming from those areas. Even Boromir can only guess, since it’s been almost half a year since he was in those areas.

The “shadow”, as used here, for me is merely another way to acknowledge the fog of war, and the word “foresee” should be read as “predict”. When you get right down to it, there really isn’t very much day to day “magic” in the story. It is only when we try to impute mystic powers into this sentence that it becomes problematic. The far simpler reading is that Elrond is using some flowery language to say, basically, “I don’t know what’s going to happen. Good luck though.”
Always after a defeat and a respite, the Shadow takes another shape and grows again.

I don't have any humble opinions.
I doubt Tolkien had a specific thing in mind but he does suggest in a few scenes in the book that will played a great part in these wars of the mind. The most obvious example I can think of is where Aragorn, Legolas, and Gimli chase the Uruk-hai and other orcs across the plains of northeastern Rohan. They struggle to follow the trail because a powerful will (presumably Saruman's) seems to be speeding the raiding party's progress and slowing down the Three Hunters' progress.

Thus, wherever Sauron's will held sway over the creatures of the land might be the geographical limits of the "shadow" to which Elrond referred.

Tolkien used "shadow" liberally to mean several different things. It's an ambiguous ubiquitous term in the lexicon of Tolkien's mythology. I don't think there is a precise definition that would satisfy everyone, so the readers will have to interpret these kinds of passages for themselves.
It is exactly The Two Towers' will scene that makes me think Elrond's 'Shadow' is a combination of Sauron and Saruman's influence; being located where he is (Rivendell), Elrond is less able to distinguish between the two than Legolas is (twenty four leagues west of Sarn Gebir).

Elrond's statement at the Company's departure then (to this reader anyhow) takes on a tone similar to Aragorn's apologies for making bad decisions.      And yes, it is tempting to try to correlate Aragorn's success rate with the boundaries Elrond cites.

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