to make the patch completely invisible, and with no shadow, you have to make the color of the patch exactly the same as the original background, something that you are not going to be able to do in Preview.
It "may" be possible if you turned the PDF, I assume that is what it is, into a graphic format and used something like Photoshop or PixelMator, both of which have the ability to exactly match colors.
I am using Preview 5.0.3 on my iMac, OS 10.6.8. In editing printed music downloaded from the internet, is there a procedure for masking blemishes on a given pdf page? I can cover the image and make the color invisible but there still is a shadow showing where the "patch" covering the blemish is. How do I eliminate the shadow?