The easiest way to do this is to try and separate the black lines from the white background. The best way to do this is to use either the magic wand or the select/colour range/shadows command. Put the black outlines in a new layer and paint in a layer underneath. If you get an effect that the outlines are too sharp and have jagged edges, simply duplicate them into another layer and use a little gaussian blur on the copy. Hopefully this will give you the result you are looking for.