Terra, Ven and Aqua all have blue eyes, but i consider them to be fixation
over Tetsuya's obsession of blue eyes......but perhaps they correlate with the
dark keyblades. The Dark Keyblades all have a blue eyes within in them.
.....I know that when Riku entered Castle Oblivion it would be apparent that he
entered through the same entrance as did Sora, but for some reason, his path
led him to a different area, that a select few of the Organization resided in.
Aside from being a nobody who can tread many paths, I'd say that either darkness,
or the Soul Eater built an alternate path for Riku. In the opening scene of KH2 we
see Sora, and Riku running on the same stair but at opposite sides. Riku is upside
down, thus it could be possible that the area he is exploring is the dark area beneath
Castle Oblivion.
.....i dunno, i'm not exactly sure how this one is playing out, all i'm getting at here is
that the Dark Keyblades may possibly open up an alternate path within Castle Oblivion.
So far the only one's with Dark Keyblades are Mickey, Riku, MX, and DS....
i dunno, i can't connect anything with the Unbirths :/