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While spraying with starch solution seems to be standard procedure, blue-black colouring seems not to stand out from a black (charcoal) surface.

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I would spray some lead nitrate solution to the surface if I didn't care about it at all. It would form nice yellow crystals of lead iodide that could be seen on a black surface. Never tried it though.

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Perhaps you have access to test stripes of paper, which are impregnated with starch and potassium iodide. In presence of molecular iodide, the water wet paper turns blue, due to the formation of a starch-iodine complex (similar to the reaction in solution, shown here.

However, it is not specific reaction as chlorine, bromine and other oxidizers may equally trigger the reaction of the potassium iodide in the paper to form the complex yielding iodine, too.

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