Setting the Black Point (Shadow)Target Values
When you set the Black Point Target values you are adjusting the image's tonal range to match the capabilities of the printer and media combination you are using. You will then select the part of the image you want to be the black point. This will be the area in the image that the first discernable maximum black is present. Due to the possible presence of reflected color in shadow areas, we will perform this step in the Luminosity Mode. Luminosity mode acts in the lightness channel of the L*a*b* color space. Setting the Black Point in this mode changes the value, but leaves the general color relationship the same. If you are sure your chosen black point in the image is neutral, it is better to perform this step in the Normal mode, however if it is not neutral it is worse. For more on selecting Black Points, see Black Point Parameters.
Go Layer, New Adjustment Layer, Curves. Make sure the Mode is set to Luminosity.
Black Point Color Picker
Setting the Black Point Target Values
Double click the Black Point Eyedropper (shadow target) Tool in the Curves Dialog Box.
This will bring up the Color Picker. You will actually be setting the black point target values in this case, not selecting a foreground or background color.
Setting Black Point RGB Values
Enter the appropriate RGB values for the printer/media combination you are using. To determine the appropriate settings utilize the Tone System Black Point Parameters Chart. Visually check to determine where you want the black point to appear. Remember, the RGB settings may not be the same value. This is caused by using the Luminosity Mode. This is OK. If you have not tested the black point parameters of your printer/media combination, you can use R20. G20. B20 as a general starting point for a high quality ink-jet printer.