Board Thread:Questions and Answers/@comment-75.23.234.106-20131230080251/@comment-16571503-20131230082700

The Azure Grimoire is an imitation of the Blazblue (蒼炎の書, souen no sho, Book of Azure Blaze) and was made to temper a power inside of it to combat the Master Unit. What the Black Beast is when the Grimoire goes berserk and starts to uncontrollably drain seithr from objects around it. To be more specific, it is born when the fusion between a Azure Grimoire (body) and a Murakumo Unit (heart) fails.

The azure Grimoire that Ragna possesses is actually broken and is the corpse of the Black Beast that appeared in the year 2100.