Starch turns blue with iodine is not a chemical reaction, it is a physical change, iodine monomer still exists

The principle of discoloration: straight-chain starch is made of alpha-glucose molecules condensed into a long spiral helix, and each glucose unit still has hydroxyl groups exposed outside the helix. Iodine molecules interact with these hydroxyl groups, so that the iodine molecules are embedded in the axis of the starch helix.