Black sole. Black flounder, also known as flounder, is tender and delicious. It is recognized as a high-grade edible fish in Japan, which is more delicious than white flounder, so black flounder is delicious. The body shape of black plaice is flat, rectangular, long tongue or oval. The adult fish's body is asymmetrical from left to right, with eyes on the left or right side of the head and a slightly protruding mouth.