Thinking about doing a halloween cake with this as the "background" color, how hard is it to get the fondant that color?

Lots of Blue and a touch of black..You would be better off airbrushing it...

Yeah I would get the fondant to be like a nice light navy color and then finish up with an airbrush.

Or use Americolor Midnight blue (I think that's what it's called!), and you'll need less. Better yet, mix it into the liquid stage when you're making your fondant - super easy. I did my "Toothless" dragon (How to Train Your Dragon cake) with Americolor and it was super easy.