This interesting phenomenon occurs when a hemoglobin molecule (the molecule that allows our red blood cells to transport oxygen around our bodies) incorporates a sulphur atom into its structure and becomes sulfhemoglobin. Hemoglobin contains an atom of iron to bind to oxygen. In sulfhemoglobin, the sulphur atom prevents the iron from binding to oxygen, and since it’s the oxygen-iron bonds that make our blood appear red, with sulfhemoglobin blood appears dark blue, green or black.
Patients with sulfhemoglobinemia exhibit cyanosis, or a blueish tinge to their skin. This is caused by the tissues on the periphery of their bodies, like the fingertips, not receiving enough oxygen (since sulfhaemoglobin can’t transport oxygen like hemoglobin can).
Sulfhemoglobinemia is caused by excessive exposure to sulphur-containing compounds, like medications that contain sulfonamides (such as sumatriptanor furosemide), nitrate fertilizer, or the overconsumption of nitrogenous vegetables like spinach (usually only in infants). Rest assured that it takes huge amounts of these compounds to cause sulfhemoglobinemia, so you aren’t risking anything by taking your prescribed medications.
The treatment for sulfhemoglobinemia is a simple one: just wait it out. Red blood cells have a natural lifespan of about 100 days, after which they’re broken down and their components recycled. So, after about 3 months, any red blood cells that contain sulfhemoglobin will have been recycled into proper red blood cells, and any non-red tint to the blood will have disappeared.