Are there any instances in literature of Blanks/pariahs serving as Space Marines? Surely they'd be unstoppable when facing warp entities. I know assassins can be blanks and they're rare, etc.
No. in order for a human to be inducted as a scout and later a marine he has to undergo mental conditioning as well as physical.
A Blank couldn't bee screened by the psykers and therefore not be eligible for becoming a Space Marine.
We see a glimpse of becoming a Space Marine in Space Wolf novels.
(Yes they are not conventional), but the process itself couldn't be that much different from other chapters.
The complete "standard" process is here.
The Space Wolves have another ritual called "passing the gates of Morkai".
In this test the Librarians take over the mind of an aspirant and judge it worthy or NOT worthy. If any other chapters have similar tests Blanks would not be permitted to become Space Marines. They certainly can't become Space Wolves.
Also we see the effect that Blanks have on psykers in the Eisenhorn and Ravenor trilogies. So any aspirant trying out for the Space Marines would be found out fairly quick and either killed outright or passed on to the Black ships.
But we have only ONE occasion of Blank in whole Wh40k literature. It is introduced in the novel Emperor's Gift from Aaron Dembski-Bowden at chapter eleven, when Grey Knights Chapter marines arrive to Titan - their Chapter monastery planet, to give away the body of their fallen brother to be buried in Dead Fields. There battle-brothers met Phlegyras - a so-called Ferrymen who’s work concludes in giving a proper ritual and burial to the fallen. These men do not serve in any kind of battle roles. Grey Knights immediately recognized Space Marine-like body type of this ferryman, and his unnatural aura of nothingness because he was a blank. But anyways, no matter questions asked, Phlegyras never revealed who he was and how he became a Blank.
At any occasion, it is impossible to say how it is possible. Gene-seed implantation to a naturally-born blank will result in immediate death because of psi-infused nature of gene-seed (please remember who their Grandfather is). At least, that's the most popular theory the imperial scholars have. So Phlegyras should become pariah after initiation to the Chapter.
Nope, it's never happened. And it never would because they're too valuable / rare / un-likeable to become Space Marines
And even if the Pariah were born on a chapter's recruiting world, chances are he would be rejected out right. Obviously the chapter's Liberians would detect the Pariah immediately and either hand him over to the Inquisition or possibly kill him, depending on the chapter.
But some chapters don't employ any psykers at all, The Black Templars for example. I'm sure they'd have other tests to check for Chaos or corruption but if a Pariah did pass these tests, they'd have another problem. Pariahs give off an aura which makes people instantly and intensely dislike and distrust them. They couldn't possibly be part of a squad when every one of their brothers permanently despises them.
Another possible explanation is that they simply wouldn't be chosen.
A typical Chapter controls several recruiting worlds which may have populations in the millions or billions. From this, they derive a very small number of recruits every year: given what we know of Chapter sizes and Space Marine lifespans and loss rates (when not appearing in other factions' codices, at least), it might be in the single digits. So they can afford to be extremely choosy, with literally millions of people passed over for every one accepted.
At the same time, because normal human empathy is to some degree psychic, Blanks are not accepted by others around them - including, we must assume, Space Marines. Even if the Blank is superlatively capable, the examining Marine will invent a flaw, simply because of his innate dislike of the Blank. Since by definition everyone who makes the shortlist to be inducted is exceptional, it wouldn't take much to tilt the examiner's decision away from the Blank.
To add to the already existing answers there is also another point that needs to be taken into consideration. The empire of mankind and especially the space marines are bound by tradition. They are bound so strongly that they often see the invention of new technology or knowledge as heresy and act upon it with fire and bolter.
Now the problem is: Blanks have never been space marines before and nobody even knows why they exist. The later probably raises hackles in the ranks of the inquisition and the former: Tradition says NORMAL huamns and PSYKER humans can become space marines nothing about the blanks......so go home and stay there or go to the administratum so that they find a better use for you.
With other words: As there were no blanks so far who became space marines it is so that because of tradition their chances are quite low to ever become space marines or even see as a possible asset to space marines.
One additional thought there: If we take into account that almost all primarchs had at least some psyker powers and even the emperor himself it could be even worse for blanks in regards to space marines as we dont know how much of their conditioning and power builds upon these psyker templates of the past.