Question

why Solar cookers are coated black​

Answer 1

Pots and pans used on solar cookers should be matte black in color to maximize the absorption.

Trapping heat energy: It is important to reduce convection by isolating the air inside the cooker from the air outside the cooker.

Answer 2

The main reason is that black color absorb heat more than any other color and it didn't loss much by reflection on their surface.