It is a pigment thing they get often times when nearing the spawn or when spawning. Odd thing is they are not permanent. If you catch one with the black blotches and put them in a livewell with white sides for an hour or longer, the spots will often disappear. Have caught some on St Clair with more than a dozen black blotches on them. Have caught them in tournaments and then at weigh in there are no signs of them ever having the blotches.