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."Why Keller keeps calling Shucorion "Shadow"?
"Another tap. “Shadow, go below and make sure the warps are powered up and the Blood core’s stable. I want to be able to increase speed without any overloads.”"
From "Gateways 2 - The Chainmail".