This seems to be the original term used by Witches for the handwritten book for Witches to record their ideas and practices. Gerald Gardner seems to have introduced the expression “Book of Shadows” as a replacement for Black Book.
Readers, please enjoy this guest blog post by Sandra Kynes, author of several books, including The Herb Gardener's Essential Guide, Plant Magic, and the new Witches' Encyclopedia of Magical Plants, among many others.
Although a few old books...