Okay, this may or may not be true, but I seem to remember that in one of the early books about the Dominion of the Black, there was mention that someone who chose to worship them/it could actually choose from a set of domains to gain clerical abilities. Does anyone know what those domains are? Or what book that was located in?
For some reason I keep thinking it would be Chaos, Evil, Madness, and Void (all of which would be appropriate) but I am not sure.
N.B.: I have flagged this to be moved to the Pathfinder (1e) forums as this is the Playtest (2e) forum.
A small number of cults devoted to the Dominion of the Black operate across the Inner Sea region. Because the Dominion has little interest in engaging humanoids on their own terms, these cults see little support from their patrons and are largely disorganized in their activities.
One especially active cult is lead by Abtaroh Vorenic, whose family has a long history of exposure to Dominion agents. She operates her cult from her family estate in Macridi.. These cultists wear distinctive magical amulets—black discs ringed by white dots, common to other Dominion cultists on Golarion.
An order of Thassilonian wizards known as the Thrallkeepers sought to conjure malignant monstrosities to impress the Runelord Karzoug. Evidence exists that the Thrallkeepers may have called forth one or more creatures from the Dark Tapestry in ancient times well before the Earthfall.  In modern times a cult known as the Night Heralds is feared to have attempted to contact beings located in the Dark Tapestry. In particular, the Night Heralds have an interest in a mysterious entity named Tychilarius.
Associated with deities granting their followers the Void Domain, deities may instead grant the Dark Tapestry Subdomain.
Source Horror Adventures pg. 49, People of the Stars pg. 23, Inner Sea Gods pg. 224, Pathfinder #46: Wake of the Watcher pg. 62, Paizo Blog - Golarion Day Subdomains pg. 1, Bestiary 4 pg. 135
Associated Domain(s): Void
Associated Deities: Azathoth, Cthulhu, Hastur, Nhimbaloth, Rhan-Tegoth, Shub-Niggurath, Yog-Sothoth
Replacement Power: The following granted power replaces the guarded mind power of the Void domain.
It Came from Beyond (Su): Once per day when you cast a summoning spell, any one creature you summon is more powerful than normal. The creature gains the advanced creature simple template. If you summon more than one creature with a spell, only one of the summoned creatures gains the advanced creature simple template. A summoned creature that gains the advanced creature simple template in this manner looks unusually deformed or hideous. This ability only works on spells you cast as a cleric—it does not work on spellcasting abilities gained from any other spellcasting classes you might have.
Replacement Domain Spells: 2nd - summon monster II, 5th - summon monster V, 7th - insanity.
I have not found information on non-Domain Classes being granted access to Domains, just that those who already have Classes that allow them to choose the Void Domain to instead select the Dark Tapestry Subdomain if they worship one of the relevant Deities. The sources cited refer to Thassilonian Wizards who sought favour with a Runelord by Summoning/Calling Creatures from the Dark Tapestry, (however this did not grant access to Cleric+ Domains).
If this is not it, good look with the search. :)