I came across this photo of "Tomato and Marble," a pastel painting from earlier this year as it was packaged up to ship to its new owner. I remembered that at the time, I wanted to post about sprinkling pastel exactly where you want (and no where else).
Cut a window flap in your b/w reference photo. Place it over the painting and open the window to sprinkle to your heart's content. See pages 92 & 93 of my book, Pastel Drawing: Expert answers to the questions every artist asks