Why douse black soil crack?

Answer 1

Not all black soils crack. The ones that are known to crack are classified as Vertisols by US Soil Taxonomy. They contain large amounts of shrink-swell clays known as smectites. Smectites shrink and crack when dry, and they expand when wet.

Smectites are also chemically active, and they readily form organo-mineral complexes with soil organic matter, which is what gives these soils the black color.

So in essence, the cracking is from the type of clay and the black color is from organic matter that is chemically bound to the clay. Black color is an indicator of organic matter, not cracking.