We recorded the initial appearance of pigmentation on the blind side in olive flounder, Paralichthys olivaceus, and investigated the morphological features of abnormal blind-side pigmentation in hypermelanic juveniles at the microscopic level. In addition, we examined the development of malpigmentation on the blind side of flounder during early development, from 24 to 94 days after hatching (d.a.h.). A total of 60 flounder were collected every 7 d and assessed for quantitative pattern changes in abnormal blind-side pigmentation. The primitive spot and the first patch were visible under a microscope and to the naked eye appeared at 38 d.a.h. (total length [TL] = 18.3 ± 0.4 mm, body weight [BW] = 57.6 ± 4.1 mg) and 45 d.a.h. (TL = 28.1 ± 0.7 mm, BW = 213.0 ± 14.9 mg), respectively, around the area between the anus and pelvic fin or caudal edge of the trunk. The malpigmented patch was composed of adult-type melanophores, xanthophores, and iridophores, indicating that it can reversely change its color according to background color, as that on the eyed side in both color and pattern. The pigmented patches subsequently became more distinctive and large in appearance and spread across the entire body on the blind side of the fish.