Hard to say. Those black things could be the shadows of the single leaves. Just to go sure -> when you use dynamic light this doesnt happen + have you placed them with the foliage tool? + do you use the default material or have you made some changes?
Because when this mesh is just a plane, it could be that the normals are pointing to the wrong way -> here you can get all information about foliage: https://forums.unrealengine/showthread.php?3268-Nature-Environment-in-UE4/page2&highlight=vertex+normal
- post a picture how the grass should look like