"The Man Who Found Out" is an excellent example of "pure cosmic horror" in that the horror derives totally from knowledge which is (in-story) so terrible that it forever blights the minds of the knowers. First Professor Ebor and then Dr. Laidlaw learn the contents of the Tablets of the Gods, and even though this information is short enough to be on two ordinary-sized tablets, it is enough to (1) induce absolute belief in its veracity and (2) utterly destroy hope in the reader. There are no monsters in the tale, nothing but Forbidden Knowledge -- which, apparently, is Forbidden with good reason.
Now to an extent Blackwood cheats. He has to cheat. Blackwood didn't know anything as terrible as the contents of the Tablets, and if he had known it, presumably the last thing he would have wanted would have been to use it to drive mad his audience. We never learn the exact contents of the Tablets, not so much as a single phrase of it.
But Blackwood doesn't totally cheat, and that's why the story is so effective.
Here is Ebor's reaction to the knowledge:
To describe faithfully the nature of this profound alteration in his character and temperament is not easy, but Dr. Laidlaw summed it up to himself in three words: Loss of Hope. The splendid mental powers remained indeed undimmed, but the incentive to use them--to use them for the help of others--had gone. The character still held to its fine and unselfish habits of years, but the far goal to which they had been the leading strings had faded away. The desire for knowledge--knowledge for its own sake--had died, and the passionate hope which hitherto had animated with tireless energy the heart and brain of this splendidly equipped intellect had suffered total eclipse. The central fires had gone out. Nothing was worth doing, thinking, working for. There was nothing to work for any longer!
The professor's first step was to recall as many of his books as possible; his second to close his laboratory and stop all research. He gave no explanation, he invited no questions. His whole personality crumbled away, so to speak, till his daily life became a mere mechanical process of clothing the body, feeding the body, keeping it in good health so as to avoid physical discomfort, and, above all, doing nothing that could interfere with sleep. The professor did everything he could to lengthen the hours of sleep, and therefore of forgetfulness.
So we see that the Tablets taught Ebor something which contradicted or made obsolete all of his old books,
sanguine, optimistic, stimulating
and his former ambitions
the possibilities of "union with God" and the future of the human race
And here is Laidlaw's.
After about a half-hour of utter horrified paralysis ...
Taking a heavy stick from the rack in the corner he approached the mantlepiece, and with a heavy shattering blow he smashed the clock to pieces. The glass fell in shivering atoms.
"Cease your lying voice for ever," he said, in a curiously still, even tone. "There is no such thing as time!"
He took the watch from his pocket, swung it round several times by the long gold chain, smashed it into smithereens against the wall with a single blow, and then walked into his laboratory next door, and hung its broken body on the bones of the skeleton in the corner of the room.
"Let one damned mockery hang upon another," he said smiling oddly. "Delusions, both of you, and cruel as false!"
He slowly moved back to the front room. He stopped opposite the bookcase where stood in a row the "Scriptures of the World," choicely bound and exquisitely printed, the late professor's most treasured possession, and next to them several books signed "Pilgrim."
One by one he took them from the shelf and hurled them through the open window.
"A devil's dreams! A devil's foolish dreams!" he cried, with a vicious laugh.
Presently he stopped from sheer exhaustion. He turned his eyes slowly to the wall opposite, where hung a weird array of Eastern swords and daggers, scimitars and spears, the collections of many journeys. He crossed the room and ran his finger along the edge. His mind seemed to waver.
"No," he muttered presently; "not that way. There are easier and better ways than that."
So, whatever the secrets of the Tablets, they are
1) Very bad news
2) about the possibilities of union with God and the future of the human race
3) involve Time
4) and Death
5) and contradict the Bible.
This list has terrifying implications. Ones which would be fulfilled by a number of Lovecraftian tales. And we know that Lovecraft was a Blackwood fan, so this (along with Chambers' The King in Yellow) may have been the inspiration for Lovecraft's own concept of Cosmic Horror. Looking at the list, I was most strongly reminded of "The Shadow Out of Time," whose revelations strongly imply that there is no God, Time is an abyss containing frightful horrors, and the human race has no future.
The other terrifying thing is that whatever the Tablets said, they said so convincingly and well that two highly-intelligent and well-educated men believed them, to the point of it utterly overthrowing all their previous beliefs. Which implies that the Tablets must be the products of a superhuman intelligence or at least lore, for only thus could they so decisively take hold of the reader's mind (we might today call them a "highly virulent meme").
The fact that Blackwood never directly describes the contents of the Tablets here acts to the advantage of the story. Firstly, it forces the reader to try to deduce the contents from their effects on the two main characters. Secondly, nothing that Blackwood could have written could possibly be as bad as what the reader might imagine -- the more so because different readers might be cosmically-horrified by different things and hence would fill in the gaps differently.
Lovecraft would go on to do something similar in his most effective tales. He, unlike Blackwood, would actually produce quotes from his various Forbidden Tomes, but these would generally be short and either cryptic, menacing or both. Note that the same technique was less effective when Derleth and Lumley produced long quotes from the Tomes, because the more the detail the less the horror.
The trick is in the balance. A horror which is entirely vague isn't horrifying either -- one must be frightened of something. The writer must decide how much to show or tell -- and how much to merely hint at.
In "The Man Who Found Out," Blackwood achieved a near-perfect balance -- due to Laidlaw's reaction.
The conclusion is also interesting. On the surface, it's a happy ending, because Laidlaw succeeds in disposing of the Tablets and forgetting their contents and even existence. But beneath the surface ...
... The Tablets are still out there, waiting to be discovered. Whatever terrible reality they described is still out there, waiting to be rediscovered or (worse) to manifest itself against us by surprise. All Laidlaw has done is retreated from terrible Knowledge into comforting Ignorance.
And the Truth shall Enslave You. Which is a very bleak concept indeed.