The blood side is sushi, and the white side is cat food?
When the tune gets to the market, the buyers insert a hollow sampling tube down into the fish. The most desired meat is the white meat(not really white until cooked, but free of blood)and the blood meat is closest to the floor. To get the $35.00/lb+ prices the tunas(blue)have to be meticulously prepped immediately after catching, and generally have someone to certify the procedure if they are going to the Japanese market.
I have seen fishermen off of North and South Carolina ruin a nice catch of yellow fins by not processing them properly and drop the price from around $5.00/lb to less than $1.00. That was twenty years ago, but I remember then that big blues sold for as high as $35.00/lb with buyers waiting at the docks to ship them in Styrofoam caskets directly to Japan.