I am a GIMP newbie, and I am creating a world map. I want different regions of the world to have distinct color palettes. However, I do not know how to Blend colors to soften lines and have, as an example, arctic grey smoothly transition into a dark green tundra. I also have a red-based region next to a green-based region; is that going to be an issue in regards to Blending?

I have tried the Blending tool, and it turned by image into a half black half white 1960's poster. I tried the Smudge tool, and it did exactly that to very poor effect. I am going to toy around with Filters and what not, but any guidance would be greatly appreciated.