While any form of vomiting is uncomfortable, black vomit is particularly distressing because of its rarity and strange colour. To a degree, all vomit is acidic because of the stomach acids that often come up with the digested food particles, so the colouration is the concern in this instance. If the problem persists you'll need to see a gastroenterologist to have an upper endoscopy performed.
Bleeding in the Upper GI Tract
The upper portion of your gastrointestinal tract consists of the oesophagus, stomach and upper portion of the small intestine, which is called the duodenum. The presence of blood mixed with stomach acids turns the vomit the dark or black colour. This indicates that a portion of your tract is bleeding. The general diagnosis for black vomit, then, is gastrointestinal bleeding.
Peptic ulcers are irritations or tears in the lining of the stomach and small intestine. Many things can cause stomach ulcers but the most common culprit is related to the overuse of anti-inflammatory drugs like ibuprofen or aspirin. Other causes include alcohol abuse, tobacco use and persistent vomiting. The stomach lining becomes irritated and wears away due to the presence of a bacterial infection in all of these cases which produces an ulcer.
Esophageal, Stomach or Liver Irritation
General irritation in the GI tract can cause black vomit. Acid reflux and a poor diet are two of the biggest warning signs for this type of problem. Minor irritation is common for many people as long as it isn't persistent. Although this cause is less serious than an ulcer, anyone experiencing something similar will need treatment from a doctor and possibly a change in diet or a prescribed medicine. Lighter coloured green to black vomit may indicate the presence of bile and some sort of liver malady.
In rare cases, black vomit may be an indicator of Yellow Fever. The viral infection is transmitted by mosquito bites and is common around the equator in hot climates like those in South America and Africa. Yellow Fever is rare to non-existent in the United States and is typically contracted by people on trips to other areas of the world. This type of infection can cause internal bleeding and organ failure. Once contracted it can be fatal.