Verified answer

Maybe trying washing it with a dark load of washing. Actually, that could be disastrous.

If you're okay with prices/doing this to your dress, you could try buying fabric dye? They're available at your local craft store, I should think. Buy a cobalt or royal blue colour, something that matches your dress. Don't go darker unless you're willing for the colour to change dramatically. They should have clear instructions on the back- follow these and you'll most likely be able to tone down the brightness and darken the colour slightly.

Just be careful, DIY dyeing can always be risky. It won't come out exactly as the packet says; you ARE putting the dye on a dark blue dress, rather than a white fabric. Prepare for the colour to be darker. Do this only if you're really desperate, or are willing to risk your dress.

Hope this helps!