Black textures on landscapes are likely because there are too many texture samplers in the material. I had a similar problem - and in my case, it was partly caused by the fact I used ‘height blend’ to blend the layers.
As a first step, try changing the blends to regular (ugly) alpha blends, and see if it works. I don’t know why it worked for me, but it seemed to help. (Duplicate your material first, as they are a pain to hook back up again!)
If that doesn’t, try removing a couple of texture sample nodes, and see if that solves the problem.
In my particular case, I kept the ‘height blend’, but used channel packing to remove 3 samplers. This seemed to make the difference.
Wish I could help more with the actual engine mechanics, but hopefully that’s your issue.