Why does iodine turn starch blue-black?

Iodine turns starch blue-black because it forms a blue complex with starch when in an aqueous solution. This reaction is highly sensitive and serves as a common test to detect the presence of even minute amounts of iodine.123

In analytical chemistry, this color change is frequently utilized as an indicator. For instance, during the volumetric determination of iodine, the disappearance of this characteristic blue color signals the end point of a titration with sodium thiosulfate.31