I need some advice! I need to know what would be the best way to get a background on a cake that looks like dark clouds (see picture)

It'll be a fondant covered cake and I don't have a air brush! This is going to be a big project for me for some "important" people so I want it to look outstanding!

How about starting with white fondant, then use black or charcoal petal dust or powdered food coloring, dry, with a brush. I'd definitely experiment and practice on fondant scraps first.

Or, you could use gel colour and vodka, and use a sponge stecial brush (new of course) And sponge it on in layers