In fact, it would seem almost the reverse. After all, it was the Dark Ones' ability to manipulate and touch T'A'R that lead to the increase in nightmares as well as It's creation of Slayer - the fact that the DO created a villain that could enter T'A'R to switch his identity argues against the idea that the DO is limited there. If Moghedien could tear a soul from T'A'R, I have a hard time believing that the Dark One would not be able to do the same.