Behind the Mythos
The Magnum Innominandum and The Nameless Mist were unified as the same entity later by other writers, notably by Lin Carter and by the Call of Cthulhu RPG (some readers still believe the Magnum Innominandum to be a term for Yog-Sothoth). For this reason, Darkness is sometimes known as "Magnum Tenebrosum" or "The Unnamed Darkness" as a parallel to its sibling.
In the The Dream-Quest of Unknown Kadath, Randolph Carter is described as falling through endless voids of "sentient blackness". It's unknown if this is related to the entity named Darkness, though much of the language used to describe the realms outside of existence such as the Ultimate Void and the Court of Azathoth relates to blacknesses, darkness, "spiral black-vortices" and such (it is also note mentioning that the Ultimate Gods are described as "tenebrous").
Lovecraft's tongue in cheek family tree: (HPL: Selected Letters of H. P. Lovecraft 4.617)
Azathoth / ___________________________/__________________________________ / / / Nyarlathotep The Nameless Mist Darkness (The ancient patrician / / gens Viburnia of Haec / / RESPVLICA.ROMANA) Yog-Sothoth------------+---------Shub-Niggurath / / / ____________/___________________ L. Viburnius Marco / / / Nug Yeb / / / P. Viburnius Marco / / Legatus of Legio 11., Cthulhu* Tsathoggua* station'd at Isca Silurum, / / in Britannia Secunda / / in A. D. 103 Shaurash-ho Yabou / / / / / / / Yogash the Ghoul Nush the Eternal / / / / / / / K'baa the Serpent Gilles Grenier, / / Lord of Averoigne / ** / / Viburnia-----------+-----------Ghoth the Burrower / / (one of the Little People) Hippolyte Le Sorcier, / ancestor of Llunwy of Wales Clark Ashton Smith ancestor of Owen Gwynedd and of H.P.L. * First of their respective lines to inhabit this planet. ** This union was an hellish and nameless tragedy.