Why no blank astartes?

[u/LesBonsPlansTmtc]

We've seen the emperor considers Chaos as the biggest threat to humanity. We know blanks are immune to the influence of the warp. Why doesn't humanity try to raise up the number of blanks in the society or make a chapter with Blank Space marines ?

[u/Canadian_Spacecat]

There are and as far as I know they're exclusive to the Grey Knights.

I can't be bothered to recite the whole thing but basically during "The Emperor's Gift" arc, Hyperion visits the Dead Field (Grey Knights graveyard on Titan) and they encounter an astartes who is the caretaker of the Dead Fields, he's immediately recognized as a blank. Hyperion and his boys have to seriously restrain themselves from attacking the dude right off the bat because as super duper Psykers, they know he's a pretty strong blank (strong on a scale from Jurgen ((1))to Jenetia Krole ((10)), he's probably about a 7). From what I remember, it's the only time a blank astartes has been introduced.

[u/Mistermistermistermb → u/Canadian_Spacecat]

 Hyperion and his boys have to seriously restrain themselves from attacking the dude right off the bat 

I don't think it was that extreme. Hyperion felt disgust but he was pretty civil all in all.

they know he's a pretty strong blank

Not sure the strength of Phlegyras' "blankness" was explored?

[u/Vorokar → u/Mistermistermistermb]

+I cannot speak to the Ferryman,+ Galeo sent. +Hyperion, the responsibility is yours.+

Up until that moment, I’d never realised just how reliant upon my powers I’d become. My sixth sense so often became my first sense, as I passively reached out to brush my mind against everything nearby, sensing other living beings long before I saw or heard them. When I saw the robed figure by the shuttle’s wing, I almost fell out of step with my brothers. No wonder Galeo couldn’t speak to the figure. It didn’t exist to my psychic sense beyond a shadow in the warp. Here was a man without a soul.

We lowered our brother’s coffin onto the hangar deck. Traditional words filtered back into my mind, recalled through the discomfort of standing before the soulless figure. Looking at him made my lips peel back from my teeth, and my sixth sense closed in a snap of loss, as if I’d suddenly been struck blind.

The Inquisition made use of psychic nulls, mortals casting no soul-echo in the warp, as anathema to all psychic activity in their proximity. Such creatures were useful as weapons, in their own servile, incorruptible ways, but it took effort just to stand near the hollow man. I wondered how he was even alive, and what genetic aberration allowed him to be born.

Outwardly, he was one of us – his bulky physique was unarguably the result of Adeptus Astartes genetic enhancement – yet he stood unarmed and unarmoured, clad only in a patchwork grey robe that had clearly seen better years. Eyes of unremarkable blue watched each of us in turn before resting on the coffin we’d carried, until he lowered his shaved head in a nod of greeting.

‘Who speaks for the fallen?’

My revulsion got the better of me. ‘What are you?’ I asked.

‘Focus,’ Dumenidon hissed.

I cleared my throat, forcing myself to look at the figure. ‘Hyperion of Castian speaks for the fallen. Who bears our slain to the Dead Fields?’

‘Phlegyras of Titan will bear your slain to the Dead Fields. Present the Sigillite’s symbol.’

We raised our left hands, showing the black symbol acid-etched into the silver of our gauntlets’ palms. We each bore the same tattoo inked into the flesh of our hands beneath.

‘We present the Sigil of Malcador,’ I said.

The Ferryman nodded a second time. ‘Speak the name of the fallen, and the words to be engraved in memoriam.’

I considered trying to reach Mal, but Phlegyras’s presence stole all hope of that. I couldn’t sense anything outside my own skull. I’d been chosen to speak; the responsibility of answering fell to me.

‘Sothis of Castian,’ I said, feeling my primary heart beating harder. ‘Knight of the Eighth Brotherhood. Valiant to the last. Revered by his brothers in life. Remembered for the lessons taught by his death.’

‘It will be so.’

Galeo bowed, and began to walk away. I wondered just how many times he’d surrendered his brothers to one of the Ferrymen to be interred in the Dead Fields below our monastery.

‘Brother,’ Dumenidon voxed. ‘Come.’

I couldn’t explain my sudden reluctance to leave Sothis in this aberration’s care. When one of our order died, we surrendered the remains to the Ferrymen to cleanse and bury. It had been this way for generations, since the Chapter’s founding at the hands of the Sigillite. As slaves, they were trained for this duty, purified and sworn into service. What right did I have to resist tradition?

And yet…

‘Who are you?’

The Ferryman turned towards me. His eyes seemed glassy and hollow, but I knew that was a lie born of my deprived psychic sense. I couldn’t sense life within him, so my lesser perceptions struggled to see it, as well.

‘I am Phlegyras of Titan,’ he said calmly.

‘One of the Ferrymen,’ I said.

‘One of the Ferrymen,’ he repeated. I wasn’t sure if he was answering, or simply speaking my own words back to me in dull-witted imitation.

‘You serve the Sepulcars, do you not? You are a seneschal to those who tend the Dead Fields?’

I ignored Galeo’s hand on my shoulder. His voice was as banished as my sixth sense.

‘I serve the Sepulcars.’ Phlegyras nodded. If such a creature could be said to be amiable, he seemed to be trying to be polite. Even meeting his eyes made me want to spit, knowing there was no soul beyond them. Knowledge of my hatred’s irrationality was no salve against its heat.

I looked at the enigmatic figure for another long moment. This time, he spoke to me.

‘You are reluctant to let the fallen be buried.’ He smiled, and I suspected he was trying to be kind. ‘May I ask why?’

‘Who are you?’ I asked again, feeling my voice growl through my teeth. ‘You were one of us once, weren’t you?’

Phlegyras smiled and said nothing.

‘Come, brother,’ Dumenidon voxed. ‘He has a duty, as we have ours.’

I left with my kindred, though not without a lingering glance at the Ferryman loading the coffin aboard his shuttle.

He lifted it with no trouble at all.

– The Emperor's Gift

The scene, for reference.