I have this lovely black sheath dress that I wear when I have no idea what to wear.
I suggest going to the mall and trying on every single black dress you see, until you find one that makes you look amazing. Do not settle for anything less. If you are on the edge, it is a no.
Fabric is really important. Mine is silk, which makes it difficult to clean and cold in winter.
Length is also really important. Depending on your age and body shape, what looks dour on one person might look excellent on another. If you are young, you should really go somewhere between mid-thigh and just above the knees. If it shows a lot up top, it should be longer.