Blood Angel may have been the author’s fictional name for something more real, the blood eagle.
From Wikipedia: The blood eagle is a ritualized method of execution, detailed in late skaldic poetry. There is an ongoing debate about whether the ritual was a fictional literary device, a mistranslation of the original texts or an actual historical practice.
Ahh...no, the error is mine alone. Thomas Harris did indeed use the Blood Eagle.
Thanks for the correction. As I was typing my mind was pondering why would the Vikings use the term angel.