Can I use Castle Black on an already standing character? Yes, you can. Neither part of the effect depends on the other (there's no "then" in between), so if either part will be successful, the action can be triggered.
Does standing a defending character remove the character from the challenge? I couldn't find commentary that said one way or the other. — Randor85 1
No, standing a defending character does not remove it from the challenge. Standing/kneeling status and participating/non-participating status are totally independent from one another. — mplain 230