I personally use the landscape tool for all my terrains 
Either you do it with landscape layer blend nodes or with the landscape layer weight nodes -> I personally use the landscape layer weight nodes because I never had any problems with them.
When you want to learn how to create pretty advanced materials, take a look at the material from the vehicle demo.
gray square -> mostly it’s because you have painted onto the component with too many layers
black square -> just select a layer - paint over the entire terrain - select another one - paint. Now you should be abel to paint correctly.