Under the skin, a black truffle hides wrapped by a black colored nori. Truffles and sushi is not common but I managed to convince my local “sushiateria’ to test it. He likes it and it does make good sense given that some fish are complementary. Most fish are not a good fit for fish given there is a conflict in flavors. I never thought that truffles (raw) and raw fish could work until I tried it. In principle the use of truffles is too abstract for many Japanese given the taste is foreign.