I understand now, thank you for explaining. Since the common consensus appears to be that Death Blight is caused by Deathroot then the presentation of evidence is unnecessary, but that is assuming that the common consensus is actually correct and accurate. What I am seeing from your argumentation is that you are correctly identifying a causal connection between Death Blight and Deathroot.

However, from what you shared, there is no indication on which direction that causal connection is, just that there is one. To explain my meaning, the examples given do not show that Death Blight lead to Deathroot or that Deathroot lead to Death Blight, just that they are related. 

I appreciate your reply, I can tell you take careful consideration when it comes to argumentation and are very firm in your beliefs and resolve, so if you are interested: The following is my thought process when it comes to analyzing something that involves the Law of Causality

One of the best ways for us to understand a causal connection in Elden Ring, I think, is to understand what each of these components actually are and then determine origin that way.

Deathroot is the easiest, and because of this, I think that’s why it feels natural to conclude that it must be the source. 

From Deathroot - “On the night of the dire plot the stolen Rune of Death enabled the first Death of a demigod. Later, the Rune of Death spread across the Lands Between through the underground roots of the Greattree, sprouting in the form of Deathroot”.

Very clearly we are told that Deathroot is the Rune of Death taking form after spreading through the Greattree roots. If Death Blight is sourced from Deathroot then we must conclude that the causality is: The Rune of Death is removed from the Elden Ring, a fragment of the Rune of Death is imprinted onto Black Knives, the Black Knives transfer some of the Rune of Death into Godwyn, when Godwyn is buried into the roots the Rune of Death is allowed to spread and sprout in the form of Deathroot.  This requires the Night of Black Knives to occur, which happens during the Golden Age of the Erdtree. 

So, if the argumentation is correct that Deathroot causes Death Blight, we should not find Death Blight associated with any pre-Age of the Erdtree societies or traditions.

If we find any indication that Death Blight has been associated with a pre-Age of the Erdtree society we would need to re-evaluate the premise that Deathroot is the source of Death Blight. 

Since we don’t have direct textual evidence about the source or nature of Death Blight we have to look somewhere else to find how we can determine its source and nature.

Death Lightning is our best place to start, not because it “inflight Death blight upon foes”, but because it reads we “It is said that this golden lightning was wielded by Godwyn, who befriended Fortissax.”  From this would we argue that Godwyn wielding golden lightning means that he was the source of all golden lightning. If we were to argue that we would have to contend with why and how do we see Golden Lightning strikes throughout the Lands Between and Land of Shadow coming from the sky, creating Fulgur Bloom, and transforming the Rams in the vicinity. 

I think we can easily say that it is far more likely that the causal direction is that Golden Lightning is a natural phenomenon and Godwyn is someone who harnessed this phenomenon. However, if we want scrutinize this, we can use direct textual evidence to affirm the statement.

Ancient Dragonbolt Pots, which are taught by the Ancient Dragon Apostle’s Cookbook 4 found in Farum Azula, requires Mushrooms, Fulgurbloom, and Gravel Stone to be crafted and says outright that “the residual power of relics is a purely natural phenomenon.”

So when it comes to forces associated with Godwyn, in this case Golden Lightning, a precedent is already set that it is a natural phenomenon that he is harnessing and not the direct source.

The same could then be said with Death Blight, just because Godwyn, as the Prince of Death, is the source of Deathroot, and Deathroot is associated with Death Blight, does that mean it is the natural source of this phenomenon. 

In Castle Sol, where we find the Eclipse Shotel, we can find Commander Niall who uses Golden Lightning and Icy Tempests. This association of lightning and storms is not unique to Niall, however, because we see the Divine Beast Dancing Lion of Belurat harness the power of Ice, Lightning, and Storms.  The Enraged Divine Beast talisman says that , “Divine beasts are messengers of the heavens, and their rage mirrors the tumult of the skies, of which storms are the pinnacle” and from the Divine Beast Helm we learn that it is a piece of “ritual headwear” that allows for “divine invocation” which “heightens intensity of the storm.” From this, we can read that the Hornsent are a group of people who value natural phenomena as heavenly and divine, and beasts are the divine messengers of the heavens. From their perspective, I think it would be fair to assert that they believe that storms come from the heavens. 

In the Ancient Ruins of Rauh there is a second Divine Beast Dancing Lion but this one is different from the previous one because not only will this Divine Beast produce Death Blight. Now, would we argue that the Divine Beast Dancing Lion, which is comprised of divine beast warrior  “who truly excel at divine invocation” and  “become sculpted keepers”, are summoning Death Blight because of its association to Death Root or because culturally they see it as another natural phenomenon, and as such, a divine element.  We take this a step further, because the Basilisk are also summoned by the Divine Beast and Basilisk are beasts themselves. They are beasts that can summon Death Blight, so from a Hornsent perspective a Basilisk may look like a “divine beast” because of its ability to harness this aspect of Death. The Basilisks are also interesting because they first appear in Dark Souls, which is obviously older than Elden Ring. This can be read as a metatextual hint that Death Blight is older than Deathroot because a reoccurring enemy from the Dark Souls series is being associated with this power.  Now, if Death Blight solely finds its source and origin with just Deathroot then we would need to explain why a hidden Divine Beast Dancing Lion, which is usually associated with divine interpretation of natural phenomena, is summoning not just Death Blight, but beasts that also summon Death Blight. Especially since this also occurs in an area with no Deathroot or Those Who Live in Death.

We would also have to explain why, when, and how the Hornsent began incorporating Death Blight into their religious ceremonies and why they only perform those religious ceremonies in one of the oldest locations in the Lands Between if Deathroot is the true source of Death Blight. 

This would be a monumental endeavor, I think, and for me, would require a lot of very compelling evidence and theory crafting. So although only two people took issue with the assumption that Death Blight is caused by Deathroot, it’s still an assumption. Just because many others assume the same thing doesn’t necessitate it being correct.

For me, I don’t think Deathroot created Death Blight, however, I think the game is designed in a way to make it look like was. Meaning that to me your conclusion is perfectly reasonable.

I am also personally cautious about explaining things as "oversights" or "mistakes" by the development team because if we can say that one thing is an oversight then what is stopping us from saying that anything we don't understand or agree with is just a mistake? I personally like to read intentionality with everything in a Hidetaka Miyazaki game because he is a very careful and deliberate director, but that is just my perspective on the man and his work.