Hemoglobin, the oxygen carrying molecule, normally exists predominantly in the ferrous state (Fe2+), which is its functional form. Hemoglobin can also exist in the ferric state (Fe3+), known as methemoglobin.
High concentrations of methemoglobin (methemoglobinemia) cause blood to appear brown and ‘chocolaty’ in color - chocolate colored blood.
Methemoglobin can occur because of heredity, exposure to drugs and chemicals.
Ramanamurty SV. Methemoglobinemia: A Reappraisal with an Indian Perspective. Medicine Update 2013. Chapter 77. Available http://www.apiindia.org/medicine_update_2013/chap77.pdf Accessed 13 December 2014.