How did you assess that your blood is dark. Usually when injury
occurs the blood will be red in color because it is arterial blood
which contains more of O2. Suppose you are giving blood samples for investigation it will be dark in color because it is venous blood
which contains more of CO2.
Those who have bronchial asthma
or a smoker may have dark colored blood because of less % of O2 absorption in the bronchial passage. Where as you say in all aspect you are normal. even though O2 & CO2 monitoring machines are available better you check up your O2 saturation in the Pulse oxi meter within a mt. you will find out your O2 saturation which should be 90 and above.