If you are using graphite and cream colored paper, its going to be pretty difficult to get "black" blacks, no matter how much you tweak levels, its still going to be using other colors. The only thing I can think of aside from changing the colorspace to grayscale (which would defeat the purpose of doing the drawing on cream colored paper, unless you want it all in B&W) is to isolate the linework by duplicating the layer with the drawing, using Select>Color Range, have it set to selected color and use the eye dropper to pick the color of the paper, which would select all of that color, delete that, then you're just left with the linework, then you could tweak that, by like lowering the saturation and playing around with the lightness and stuff like that. But its highly destructive since you're deleting elements, albeit on a secondary layer. But even so, like I said, without going into grayscale, its still going to have other colors incorporated and not going to be pure black
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