Hi everyone ! I post this topic again, the first one is on the “Help Me!” forum.
I finally managed to create a macro which allows you to modify terrains in order to remove the seams :
http://terrafgen.free.fr/Downloads/Macros/Blending_Macros.zip
The macro takes 4 inputs and create 4 outputs that can be placed side-by-side.
You can of course choose the blending size.
You can see an example here, the 4 outputs have been merged into a single big image :
http://terrafgen.free.fr/Downloads/Final.jpg
Tell me what you think 