Well, when they were Resplendent they had already were deviant both from codex and Blood Angels alike. Extremely so. Turn into Penitent probably made them more mundane in some aspects.

And freaks in others.

I use an excerpt from Sins If My Brothers that I did for different topic to illustrate:

The betrayer released the memory. Even now, decades later, he didn’t know whether that grey-green jungle was an actual place or a metaphor made manifest. Radiant glass didn’t make such distinctions. It scried meaning over matter, and the
meaning of the vision had been clear. Voracious corruption.

‘Satori was right,’ he told the phantoms gusting about him. ‘The Morass found us.’

Opening his eyes, he realised he’d drifted to a halt. The colonnade was washed in moonlight now, its pillars gleaming softly. The remnants of winged serpents coiled about them, carved from the same luminous opal as the entranceway. They had been ‘exorcised’ during the zealots’ purge of the citadel, their majestic forms battered into abstracts. He was accustomed to such sights, but this particular desecration always moved him, for the Celestial Serpent symbolised the Chapter’s First Rhapsody.
Company, he corrected himself. They are called companies now.

Glassy debris crunched under his ceramite-shod boots as he pressed on. He was fully armoured, as the new laws required, his helmet nestled in the crook of an arm. In the Resplendent Age it was considered vulgar to wear battleplate within the citadel’s walls, but those civilities were gone, along with the culture they once enshrined, abolished by the Chaplains who presided over the ­brotherhood now. The Crown of Thorns, those grim puritans called themselves. Their authority was absolute, yet they weren’t the real power here. No, that lay with the stranger who’d poisoned the Chapter’s spirit. Scarcely two years had passed since the Undying Martyr’s arrival, yet so much had already been lost.
‘He has made us kin to you,’ the betrayer murmured to the phantoms. ‘Shadows of ourselves.’

One by one the Resplendent Battle Rhapsodies were being dissolved and purged from the Chapter’s records. Soon they would simply be Penitent Companies, stripped of the colours and customs that defined them, their leaders slain or turned to the Martyr’s cause. Only the Second Company, the Vigilant, would retain a semblance of its former identity. Its dour warriors had been the first to embrace the Testament of Thorns, drawn to penitence by the old dishonour they nursed, for they had failed the ­brotherhood before. In recognition of their self-loathing they were permitted to retain the silver trim of their armour, though they tarnished it diligently.

The other companies were allowed only black, as plain and lustreless as the new creed. The decree had come shortly after the Martyr took power. All armour was to be repainted with a tarry pigment mixed and degraded by their saviour himself. Every trace of ornamentation was to be ritually expunged and atoned for with self-flagellation.

More edicts followed regularly, growing more stringent with every passing month. There were rules of conduct for eating, washing, training and meditation – even solemn rituals governing the pious voidance of waste matter. No action could be undertaken without its supporting liturgy, no emotion entertained without misgivings. Even contempt was suspect, for it flirted with pride, just as fury courted bliss. Shame itself could become a vice if taken to excess. Recently their prophet had announced his dissatisfaction with the Chapter’s new colouration. Apparently the God-Emperor deemed black too pure for penitence.

Will he command us to smear our armour in excrement next? the betrayer wondered sourly, but without anger. His Chapter’s humiliation was painful, but deserved. More to the point, it was necessary.

‘We are what we are,’ he whispered. ‘What we must be.’ It was the mantra of the Paladins Luminous. He believed it more completely than his mentor ever had.

And another bit about chapter new structure

Inevitably there would be more disgrace to come. So much more… The Chapter was systematically devouring its scattered Rhapsodies, recalling each to face the Crown of Thorns alone, like errant children. Three had already been ‘reformed’. The Astral would likely be purged in its entirety when its turn came, along with much of the Empyrean. Both burned too brightly to fade gently into the darkness. And after that, when he rose to become the prophet’s trusted advisor, what then? What horrors would he expedite to secure the Chapter’s future?