It wouldn't be anything you ate or drank, even if it was black to begin with, as the acid bath that is the stomach decomposes most stuff to a point where the colour it originally was isn't really visible. It's most likely to be the lining of your stomach - gross but true. It could either have been the sheer amount you drank that caused some of it to be eroded away, or that you had some kind of a reaction to a particular ingredient. Either way, eat only bland food for today, like chicken, fish or white rice (if you can bear to eat anything!), avoid anything sugary or acidic like fruit, fruit juice, plus the obvious like chocolate and sweets, and if you want to take a painkiller, stick to paracetamol - avoid anything labelled NSAID (the most common is ibuprofen) as these will irritate your stomach further. Drink plenty of water and get a good night's sleep tonight. You probably haven't done any permanent damage but if you're still feeling ****ty and throwing up by tomorrow give your doctor a call.