Overlay issue on map image

[DoblyTufnell]

Hi

I am following a tutorial (link below) on creating a fantasy map.. The chap in the tutorial is using Photoshop but says this stuff should work with GIMP.. Alas it's not. 

The tutorial is here. (note he has better drawing skills than me) }}

http://www.fantasticmaps.com/2015/02/how-to-draw-a-map/


Under the heading "Block in the bulk shadows" he says.. 

"Create a new layer and set the blend mode to overlay. I then pick a hard round brush with size set to around 20-30px, and opacity set to pressure sensitive. This means that if I press lightly, I get an almost transparent line, if I press hard, I get a deep opaque black. Oh, and set the brush colour to black.

I do this.. 

When he does this is paint brush makes the area behind a forest darker. It makes the brown background look darker there. 



When I do it, it paints shades of black on the image. 

Bound to be something obvious I am missing here. But I sure could use some help here.