In the Book of Shadows (pg. 16), the Umbral Strike spell's details read "School necromancy (shadow) [darkness]". I've found no other spell with Shadow as a subschool of Necromancy, only of Illusion. As "Book of Shadows" is a rather minor Player Companion, I've not seen any mention of this in FAQs/errata.
Is this supposed to be...
"necromancy [shadow, darkness]"
"illusion (shadow) [darkness]"
exactly as it is, with "Shadow" added to Necromancy's sparse subschools