Black soil means it contains coal and it shoud be black in colour. But that soil is also contained the coal in which cotton grows and we say it as alluvial soil
Black soil does not nessisarily mean the soil contains coal. It may contain a high level of organic so as in alluvium, it may contain a high level of ash from volcanism, it may have a high content of charcoal from fires. So, there are many reasons for soils to have a rich dark color. Because of the high organic and mineral content of the soil, it makes for a very rich soil so is good for growing cotton and many food crops.
By the way not all coal is black. Some is brown.