Open your image in Photoshop. Open levels or curves and click on the black eyedropper. Then click on anything in your image that should print as total, featureless black. That sets your black. You can also use the white dropper to set your whites, and the neutral dropper to set your neutrals.
But you'll quickly find this doesn't provide reliable results.
A more cumbersome, yet more reliable method is to open your image, then open levels. While holding down the Alt key, drag the far left arrow (under the blacks). Your image will turn all white, then as you slowly drag the arrow towards the right you'll see areas appear as black. Those will be the areas that print as featureless black.
If you're heading where I think you're heading, cow spots are not actually black (at least not as far as photos are concerned)