0. Cyanosis is the abnormal blue discoloration of the skin and mucous membranes, caused by an increase in the deoxygenated haemoglobin level to above 5 g/dL.. Cyanosis is the abnormal blue discoloration of the skin and mucous membranes, caused by an increase in the deoxygenated haemoglobin level to above 5 g/dL.