Option 1: get ultramarine blue deep. It's darker than ultramarine. However, there is an easy work-around to darken regular french ultramarine.
Option 2: Ultramarine is a dark blue with a tiny reddish tinge. Try adding a touch of dark-slightly-yellowish red (eg, burnt sienna) to some ultramarine. You'll get a predominantly-blue-with-a-tiny-bit-of-blue/red/yellow in it. Blue/red/yellow equals black. So essentially you're mixing black with your ultramarine.
You do not want to mix in a tube
black (ivory, mars, etc black) because it will "deaden" the color. Instead you are adding a "chromatic" black made by mixing red/yellow/blue. Mix tiny quantities of each color until you get the proportions figured out. Another chromatic black is approximated by viridian plus a dark red like alizarin or cad red deep, or by prussian blue/alizarin/burnt sienna.
...Or you can buy chromatic black in a tube, but I never saw the point when it's actually pretty easy to mix on the fly.
Hope this helps. Good luck!