The term meaning pertaining to the formation of blood cells is

Hematopoiesis

Explanation:

Hematopoiesis is the process of blood cell formation and this occurs in the bone marrow. Hemato - is Greek, which means blood. - Poiesis is Greek, which means, creation or production. The bone marrow produces some white blood cells, all red blood cells and platelets. Its purpose is to produce blood for the living organism and replenish blood lost.