Some species of mollusks have dark green tinted blood rather than red or black blood because of a property called Hemocyanin, in which copper is used instead of iron to transport oxygen in blood. But how could you explain a human who has green blood? It happened to one man in Vancouver as doctors discovered shortly before he went in for surgery that his blood had changed to a dark green tint somehow.
When sulfur is incorporated into hemoglobin, it turns green. And the way the patient got sulfur in his blood was due to a rare disease called sulfhemoglobinemia. Sufferers of this incredibly rare disease have hydrogen sulfide combining with ferric ions within the blood. As a result, oxygen is not transported in the blood and the result is eventually death unless treated.
In 2007 a man was admitted to an emergency room after he fell asleep while in a sitting position and cut off circulation to his legs. The 42 year old man was admitted to St. Paul hospital and under the guidance of Dr. Stephen Schwarz and Dr. Giuseppe Del Vicario as well as Dr. Alana Flexman. When nurses withdrew some of the patient's blood from his legs they were shocked to find it had a distinct dark green coloration within the syringe. The cause of the mysterious colored blood was eventually discovered, and the culprit was found to have been taking of too much sumatriptan for his migraines.
Actually, believe it or not everyone has blue blood in their bodies as the deoxygenated blood is taken through arteries toward the lungs where they are replenished. Deoxyhemoglobin molecules, which are often found in the body are responsible for the blue veins seen in most people. Of course this blood turns red as it leaves the body, so even if you get a paper cut on an artery you will still not be shocked by bluish looking alien blood.
In the classic series Star Trek by Gene Roddenberry, Science officer Mr. Spock would often receive criticism from the medical staff for being a "green blooded" Vulcan. The pointy eared Vulcans, to which Spock owed half his heritage, were Hemocyaninic and therefore had copper in their blood as opposed to iron. It's the same property that turns some blond hair green if bathed in water with old copper pipes. As the copper oxygenates, it changes hue (similar to an old penny when left in water for a long time) and as the green particles adhere to hair, they can even stain the hair for up to weeks afterward.
Doctors suggested that the bizarre condition would eventually cure itself in some cases, but in others a blood transfusion would be required in order to replenish the oxygen rich blood and remove the excess sulfuric blood. In the case published in Lancet journal by those treating the Canadian patient, the patient survived his surgery. It seems to be a sign of the times now that we actually have drugs now that will turn the blood green if taken incorrectly.