Smudge tool is not blending correctly.

Hi!

I have recently started to use Photoshop CS5 and have realized my smudge tool is not smudging like it should be,

Here is an example:

What I have learned is if I put a solid color under the art or line I am smudging, it returns as it should be, like so:

   I really want to smudge lines without having to put a color under it.

   I do use a tablet but I am having the same problems when I use a mouse. I also use a PC, not a Mac.

   Please help! I've been trying to get rid of this problem for ages.