Overlay Blending-Mode in Blender different from Gimp?

Hey Folks,

I tried to create a feathered sphere as a texture by creating a quadratic sphere and overlaying it with simple clouds.

Here what I get in Gimp (and what I expect):

And here what I get when I try to re-create the result as a Blender procedural texture:

I have attached the blend and gimp files:

overlay.zip (142 KB)

Now the question: Is the overlay mode implemented differently in Blender? Or could this be a bug?

Edit: In Blender compositor overlay works as expected. The only trouble seems to lie with materials…