Any matt fiber is a breeze compared to RC. You dilute the spotone and test on the edge of the paper (or a scrap/test strip of the same paper). I don't try to match exactly, I go lighter than the surrounding and repeat gingerly as necessary.
However, if its something more than just a small spot or blotch, I would probably choose to fix it on an intermediate rather than a final, either through spotting or local bleaching. You can do it over and over again till it looks acceptable, then have your master to print from.
Richard's suggestion works if you sandwich his transparency with an internegative, or use your print with it to make the internegative. But if it were me, and I was short on time, I'd throw the print on a light table, pencil in the density on the back, then contact print it twice in succession. The print will also take on the texture of the paper, but that can also work in your favour vis-a-vis obfuscation.