I would strongly suggest that you make a test cake and icing before your party since very dark colors tend to taste funny-not-in-a-good-way, and to stain the tongue and teeth.
You could cut your decorations out of dark chocolate and apply them to your frosted cake. Melt the chocolate, pour out into thin sheet, cut out before completely set, then let it finish setting before use unless you want the shapes to be curved.
Maybe for the cake itself you could add some black food coloring to chocolate batter to make it darker?
Yeah I do have to agree with erikau. sometimes black fondant turns out really awkward. Maybe you could try using a milk chocolate fondant, then dye it black. It's easier than working from white fondant. That would mean a better taste, less black goop on peoples' teeth (but still some goop...), and a better color. With my past experiences with it, dying white fondant black usually gives me a greenish color.
Also, I'm thinking for the marble you could just get a nice marble cake mix set from a local store, probably vanilla & chocolate (usually the chocolate looks pretty dark, like a black to match your theme) marble mix, just follow the instructions on the box, but for a moist taste you can add 1/4 cup more than the box calls for or it you want to be more healthier, try substituting apple sauce for the oil the box calls for.
Good luck with your cake!