BFFS!: No actual effect. Succubus and its damaging aura are only visually larger, but the damage is neither increased nor dealt any further away from the familiar.
If Succubus is spawned through Cambion Conception, its dark aura will not grant a damage multiplier to Isaac.
Bug!
If Sacrificial Altar is used to sacrifice Succubus while Isaac is in its dark aura, the damage multiplier will remain active until the remainder of the floor.