Whether you're painting your walls, children's furniture or a canvas, darkening paint requires a general knowledge of how to mix colours. To make any colour of paint darker, you must add a certain amount of black paint and mix it together. Blue paint is already a dark colour on the spectrum, and so a greater amount of black paint must be used (unlike a lighter colour like yellow would require). Darkening blue paint is moderately easy process, but requires a lot of time and patience.
Buy an equal amount of both blue and black paint. Remember that the darker you want the blue to be, the more black paint you'll have to mix in with it.
Take a bucket, or empty container, and pour some of the blue paint into it. Then add a quarter of the amount of black paint. Use a stick to stir the paint, and after a minute or so you'll notice the blue becoming darker. When you start mixing, you'll first see swirls of blue and black. Once all of the swirls have dissipated and you develop one uniform shade, that means you're ready to paint. But do not apply this paint to anything until you have mixed it together completely.
Add more black if the shade isn't dark enough. And never use too much paint in one mixing, because if you accidentally make it too dark, your paint will be wasted. You can add some white paint to lighten something that you accidentally made too dark, but use a tenth of the amount of white paint as you would black. Using too much white paint in the mix could turn your colour greyish.
Use this three-step process when painting smaller things, because trying to match the correct shade of dark blue multiple times is a challenging process. If you're planning on painting entire walls by using this manual mixing process, realise that you're in for a very long day. It is always recommended to buy the paint pre-mixed for the exact colour you want for your walls at the store. This will save you a lot of time.
Before you begin mixing, make sure that you're wearing clothes that can get stains on them. And make sure your work area is prepared for spills (lay down newspaper to be safe). Mix paint in a well ventilated area, inhaling paint fumes in an enclosed area could cause you to pass out.