Did the color migrate just from wearing (including going in the water) or was it from washing and hanging to dry? Regardless, a swimsuit needs to be able to air dry without the color from one section bleeding into another. Your swimsuit sounds defective, and you might want to consider returning it to the manufacturer for a refund. As far as removing the blue color from the white belt, don’t use Clorox® Regular-Bleach! Most swimsuits contain Spandex (also called Lycra and Elastane) which will lose its ability to recover from stretching when bleached. You could try Rit® Color Remover. Follow the package directions carefully, and work quickly so you only remove the fugitive blue dye from the white part. Of course now you will have a wet swimsuit that can bleed again. To help prevent additional bleeding, start by using a hair dryer on the “cool” setting to blow dry the sections where the white and blue fabrics intersect. You can then lay it out in the sun to dry quickly.