Black and White Mix
When you convert photos to black and white in the histogram, this section becomes enabled, replacing the HSL/Color section. In it you can adjust the overall image lightness of the converted photos.
Use the Lightness sliders to adjust the brightness of the original colors, which are now black and white.
Use 
to adjust the lightness of a specific area of the photo. Just click on the area and then drag the mouse up to increase the brightness, or drag it down to decrease, making the area darker.