Hi ,
The black splotches are a known issue that is currently being looked into. To correct them, what you will have to do is paint over the entire landscape quad with a single layer of paint, then you should be able to proceed as normal. If they continue to crop up, you may have to cover your entire landscape with a layer then paint. Additionally it should be noted that you should have at most 9-12 textures (including normals, roughness, etc) on any quad of landscape. Any more will cause it to revert to the default texture until less textures are present. This is being looked at to be increased to about 32 textures but I don’t have a timeframe on when that would occur. The landscape tool in editor does have a few quirks, but once you get used to it it is a very powerful tool that can put together some remarkable scenes. Here are a few tutorials that may help you in your future development: