Melena (from melaina, feminine of melas, Greek for 'black') is dark, sticky, tarry stool caused by the presence of digested blood.
This is usually due to a bleed, either fast or slow, in the upper intestinal tract, or a slow bleed in the lower intestinal tract. In either case, the blood has to have had adequate time in the intestine to have been broken down. Not only is this stuff sticky (tarry does not only describe its appearance, but also its consistency) and damn near impossible to clean up, but it also has a unique, horrible, very memorable smell. Its smell is rather sticky as well and has a tendency to linger in the nostrils for a VERY long time. It is incredibly pervasive, so if there is one patient with melena on a floor, the smell will be everywhere.
The counterpart to melena is hematochezia, which means bright red bloody stool. Especially lucky people may have both, which leads to passage of nasty smelly blood with what looks like large clots floating in it.
Fortunately, I've only had one patient with this so far, as it is extremely nauseating. I've had waaaaay more experience with this than I'd like. It's the one smell you never get used to.
Melena may also be called melenorrhagia or melenorrhea.