How to paint on a height based landscape material?


I have a problem with my hight based landcape material. The hight based system works great so I added additional layers so I can paint on the landscape more details. If I just paint the Dirt Layer it also works great but if I start painting the Gravel layer in the same landcape square as the dirt is painted in it turnes black even if they are not overlaping.

→ 2 painted layers in a landscape square => the square gets black.
How can I solve it?

Thank you very much!