Primer would be a good idea. You can have the primer tinted same color as the paint you are going to use, then maybe you will only need one coat of the new color.

You will have to use a good quality sealer/undercoat to cover the black, before painting the finish colour.

I'm looking to paint my room a peacock blue or electric blue. My walls are black though because, my "Gothic" brother had the room before me.

My Mom says we might have to buy primer first. Will we need to? The blue is a dark type of color. Medium actually. So do we need a primer?