Usually really dark to black stool is a sign of high iron in you system. When you are absorbing iron it can turn your stool green, when there are high amounts of iron it can be very dark to black. If you are worried or have other symptoms that are worrisome with this consult a doctor but, just dark poo usually isn't serious.
Some factors that would lead to dark brown stools include bleeding in the digestive system. The blood would turn dark red after getting exposed and which may cause the stool to have such a color. Ulcers and cancer also cause dark brown stools. Read http://www.ehow.com/facts_5569914_dark-stool-mean.html.
Dark brown isn't anything to worry about. Black or nearly black is usually an indication of bloody in the stool, which isn't normal. If your stomach was sufficiently irritated by alcohol consumption, it could have been bleeding which would cause black stools. Probably a good idea to see your doctor. And there's no need to feel embarrassed to talk to your doctor about it; he/she has heard it all before at least a few times.