In my recent painting Canal at Dusk, 30″ x 30″, I avoided using blacks in the deep shadows. Instead I opted to mix two extremely dark yet highly saturated colors, Alizarin Crimson Permanent and Thalo Green, to create my darks. Using two deep colors to create a black allowed me to bias the color mixture to get rich, vibrating hues of either warm or cool variations.
If you are painting in the field and you need a deep, dark color to quickly add to other pigments you might consider using Chromatic Black which is a combination of Thalo Green and Quinacridone Red. This mixture can be quickly made cool by adding viridian green or warm by adding alizarin crimson permanent (or any color you desire). Another favorite of mine for making black is Dioxazine Purple and Transparent Earth Yellow. This mixture, when applied half mixed, creates an exotic and rich color.
Black, like all other colors, will appear more vibrant if presented as broken color, or at least have elements of the basic components of other colors. For example a green will be more vibrant if it has flecks of blue and yellow sparkling through it. Just because an area in your painting is dark doesn’t mean it shouldn’t vibrate with broken color. And using half mixed, complementary colors will make this vibration easy and satisfying to accomplish.