The term black people is used in systems of racial classification for humans of a dark skinned

Human skin color

Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...

 phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

, relative to other racial groups.

Different societies apply different criteria regarding who is classified as "black", and often social variables such as class, socio-economic status also plays a role so that relatively dark-skinned people can be classified as white

White people

White people is a term which usually refers to human beings characterized, at least in part, by the light pigmentation of their skin...

 if they fulfill other social criteria of whiteness and relatively light-skinned people can be classified as black if they fulfill the social criteria for blackness.

As a biological phenotype being "black" is often associated with the very dark skin color

Human skin color

Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...

s of some people who are classified as 'black'.