Well, sounds pretty cool. But titan bubble being black would have an unfair advantage hiding who is inside. Like you can see Helm of Saint 14 so you know not to go in there. but if it was black, you couldn't see. But other than that, no prob with this idea.
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I would imagine it would be the same as a normal titan bubble: having a lighter, transparent colored "nucleus" with a more saturated highlight surrounding it. In this case it would be a transparent grey center with a black outline. : )