Human blood includes two categories: arterial blood and venous blood, of which arterial blood is bright red in color, however, venous blood is mostly dark red blood. As for the dark color of blood is what is going on, it may be drawn from venous blood, while for some myeloproliferative diseases, such as true erythropoietic disease, it may cause an increase in red blood cells, sticky blood, and dark red color. There are also cases of sticky blood lipids, which can also appear slightly deepened blood color; there are also cases of primary thrombocytosis and myelofibrosis, which also have deepened color, so it is important to first look at what disease is causing it.