Just a quick little sketch with some ivory black. I never use ivory black. I've always preferred neutral tint or burnt umber or sienna mixed with ultramarine blue for more organic blacks and greys, and Payne's Gray for deep, absolute blacks. But, this reference photo was screaming for a grayscale treatment and what better way to do grayscale than with a pigment with "no color"?
And, for general FYI, there are many more ways of mixing greys and darks/blacks including three primaries or two complimentary colors. UM blue and burnt umber/sienna are essentially complimentary colors for the purposes of mixing grey. Using two complimentary colors seems easier to me than three primaries. I try to limit all my palette mixes to two colors as much as possible anyway. And, a two-color grey is easy to warm or cool as necessary. Sometimes I even get the added bonus of a little pigment separation on the paper during drying. I think the look of UM blue and earth tones separating is one of the reasons I picked up watercolors in the first place.
So, what's ivory black good for? Mixed with a little cool yellow, you can get some nice greens.