Just so you know it will take a LOT of gel color! It's a combo of Royal Blue and black. Color your fondant to a dark royal blue, then add black until it gets to the right shade. And don't forget to wear gloves!
If you go to my photos, on the second page I have two cakes with dark blue- the Mouse & the Moon is a 'Midnight Blue', while the tiered Moon & Stars cake is more of a Royal Blue.
HTH
If you have satin ice, in blue, add some black fondant till you are at your prefered darkness... if it still is not quite the navy you want add a little royal blue color gel
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