Why does plain paper turn black after a few drops if iodine are applied

When iodine reacts with starch, it is trapped in the starch's helical structure forming a blue-black precipitate. Iodine is not very water soluble, so in making an iodine solution, potassium iodide (KI) is added. This results in the formation of a triiodide complex, which is completely soluble in water.

it will increase due to the time span

this protocol provides a detailed procedure for co-immunoprecipitation (co-ip) to identify protein–protein interactions.