I am getting black screen on iOS 8 device. Launch screen from LaunchScreen.storyboard works fine on iOS9 device.
I am pretty sure you did not need a launch image asset for iOS8 until I upgraded to Xcode 7
After much head banging, I found out that I had tried to attached a Cocou Touch file with my LaunchScreen View Controller. That was causing the problem. Once I removed the all outlet references from my launch screen, it started working in iOS 8.