“Black” vomit is usually a sign that there is blood in the upper end of your GI tract – stomach to you. If it comes out the other end (as tarry stools) then the bleeding is generally lower down.
Whereabouts it is depends on the consistency of the black stuff in the vomit. If it is like coffee granules it is slightly further down (and has been digested a bit more) and if it is like sludge it is at the top end.
Most times this indicates a perforated gastric ulcer or something similar. It is unlikely to be anything you ate – unless you did a vampire impression last night and drank a cupful or two of blood.
Best thing is to get to the quack ASAP. If you have stomach cramps or if it happens again over the weekend – zip off to your local A&E.
BTW take a good book ‘cos you’ll be waiting for hours.