Pretty much like Blazer73 said. You can pick up a pint of code L2 Ford color and only spray a couple of coats over your sanded base black.
You can also have an auto supply paint store mix you up a pint of straight blue pearl toner, then mix it with some binder or clear basecoat (if you use the PPG Global system, this is D995). Have them mix it about 50-50 with the blue pearl.
Spray a couple of coats over your sanded black base.
Go easy with the latter method. It's very easy to go overboard with the stuff, since you can't see the spray on your masking paper. 2 to 3 coats should do it.
You might want to do a test panel first just to make sure you get the effect that you want.
