Here you can see two pages from my sketchbook. I test the colors on the left side, and on the right side I have those which I used in the painting.
Another way to make black is to mix blue with brown. Ultramarine blue mixed with Burnt Sienna gives you great black, but even better gray. I chose to mix Indanthrene Blue with Burnt Umber. This mix gave me quite a strong black which I used to paint the ends of the stemens.