It takes a lot of navy blue gel to make navy blue fondant! I covered this cake board in navy blue fondant by using navy blue and black gel color from Americolor. I used Crisco on my gloves to keep it from sticking and I made it 2 weeks before I needed to use it in order for it to have plenty of time to rest! Good luck!
I'd use both blue gel and powdered blue color, mix in a little and let set for a while then add more color. You can also add lecithin to the fondant to help hold the color.
It will get darker as it sits, too. It's a hard color to get to using liquid colors...I use powdered food color, or sometimes mix modeling chocolate that's already blue (made from dark blue candy melts) with the fondant.
What color fondant are you beginning with?
I've learned (the hard way) to start from something close to the color I want to achieve and it's especially true with darker hues. The less color you have to add, the less chance of the stickies.
Also, when I (used to) make my fondant, I'd add coloring to the wet stuff before working in the PS. That way, you're not working yourself to death trying to color a batch of white