Question

why most of the substances turn black after being burnt???

Answer 1

When an object is burned, such as wood, chicken, etc., the object turns black because of the carbon residue left behind. Organic materials are largely made of hydrocarbons which, When subjected to a high energy source, split apart into their counterparts.

Answer 2

It is generally the organic compounds which turn black on being burnt. Organic compounds contain carbon as an important element. On heating the carbon residue which is left behind makes the substance black.
Hope this helps.