The shinigami appear to be based on Youkai, specifically bakeneko.
Bakeneko survived on the poison of an unknown snake, given that Hollows require the venom of other Hollows (or shinigami) and the shinigami require rhododendron honey.
In Ethan Rayne's Very Bad Day, Youkai were said to be able to heal any wound very quickly unless the knife was licked by someone full of hate. When Isane was munched on by the Hollow, the saliva kept her wound from healing.
