Again, it's a reference to the first Challenger book. Shucorion is a Blood; he has dark blue skin and wears dark grey. On p. 130, Nick thinks of him as "a shadow come to life."
By the way, you forgot to implement underlining and bolding code as you did in the other copy of your post:
He became First officer, so he was shadowing Keller's every move? And yeah, dark clothing? Also alliteration? Sh... shadow, Sh... Shucorion?
Shocked to learn that the woman who wrote a Star Trek story justifying Japanese internment would write a story with some casual racism in it. Shocked, I tell you.
"World of Strangers," the second tale in the Star Trek: Enterprise Logs anthology (the first two of which, both by Carey, were straight historical fiction about real Enterprise captains, this one being a captain of the WWII aircraft carrier of that name).