How long until I can add blue into my hair, after dying it fully black today?

You should wait about a month, however blue wont go over black. The parts you want to be blue, you'll have to bleach them first and then add blue. Plus, you'll probably have to bleach it twice, so you're going to need to wait longer since bleaching can fry hair if you don't do it right. Right now your at a 1 and you'll need to be at least at an 8 if you want the blue to stick. (1 is the darkest on hair color scale and 10 the lightest).