What color do you get when you mix dark blue and light blue?

BLUE!!!!!

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The color (hue) will be a blue, but it depends on how your mixing what color you finally get. If mixing subtractively (like inks on paper) your colors are filtering the light that reflects off the page (we'll assume the light and the page is white). In this case "light blue" is a transparent blue and dark blue is blue and black, so the white light in the room will be both filtered by the pale blue and the dark blue. The pale blue removes some of the light which isn't blue, the dark blue removes a lot of the light - even some blue. None of this puts any light back, so the dark blue would dominate - you would get dark blue.

If you were mixing light (additive mixing), dark blue is just a small amount of (dim) blue light and light blue is blue light with a bit less of all other colours in it (white). What you get then is the light blue, with just a little extra blue in it. Imagine a room in daylight and switching on a blue light-blub - would you notice the room becoming more "blue"? You'd probably still call it light-blue.

If mixing opaque paints and you took a pale blue (blue+white) and very dull blue (blue+black) you would get a cool-blue-gray. You certainly wouldn't get back to a spectrum (saturated) blue.