I had the same problem on my Vizio 4K Smart TV and have found a solution . For some reason my backlight had to be below 60 . I don't understand it but setting it below 60 worked . Everyone who has this problem should try this . You never know .
Re: Black Screen Flashes With Ultra And Samsung TV
I have had the same issue from the day I baught the Roku. The only thing that worked was downgrading to 720. New HDMI cords, and all the setting recommendations did nothing to get rid of the black screens. I found this also happens on both my ultra and streaming stick. To make matters worse is the TV never detects a signal unless I cycle power each time I watch, unless I use 720.