One of the five ancient tomes authored by Zet, an ancient draconic deity of forbidden knowledge. The book is bound in blue dragon-scale leather and is imbued with evocation magic.

When attuned to the book, you gain resistance to Lightning damage. You gain +1 to all spell attacks and damage rolls. The tome has 5 charges. You can expend a charge as an action to cast the Witch Bolt spell at level 1. You can expend additional charges to raise the level of the spell. When you cast Witch Bolt using the Blue Tome at higher levels, the damage scaling also applies to the secondary effect of the spell. The tome regains 1d4 charges at dawn.