검다니 is composed of 검+다니 검(다) means "black, dark" 다니 is a suffix used to express that certain fact or situation is amazing or unbelievable (I can't believe that <V/A/N attached>) 피가 검다 means "blood is black" 피가 검다니 means "I can't belive that blood is black". For more explanation, refer to http://keytokorean.com/classes/intermediate/intermediate-grammar-23-%EB%8B%A4%EB%8B%88%EC%9A%94-really-i-cant-believe/