For me, I don't want to contribute to any suffering, so I don't use animal blood/bones even though they are found in nature. The stuff you can buy is generally from factory farming operations, or something just about as bad. The more I buy, the more they'll create. I don't want to be part of the demand/supply equation for animal parts, even to any small degree. But I agree, animal blood and bones are healthy for plants, and can definitely be found in nature! I think plants grown using such products are still organic, but perhaps not necessarily vegan.
Thanks for asking :)