I know this thread is rather old but I have the answer to this issue.
The reason it turns Black is because you are probably using Shaders that are most likely too complex to be supported by Mobile Devices and the result is that it doesn’t render properly therefore the output is black.
You can see this per say if using a Moving Shader (panning with noise nodes,add, powerx, etc) created in UE4 and it is set to Fast Gradient, yeah it renders faster but suffers in this way since it is highly unoptimized.