I am also having the black screen issue with the Roku Display Type set to 4k HDR 60hz with a brand new Vizio PX65, I did not have this issue with a Vizio M60.
If I manually switch the Roku Display Type to 4k HDR 30hz, the black screen issue goes away, but any motion on the screen looks awful.
4k 60Hz also does not have the black screen issue, and looks ok.
So I agree with others, this is an HDR issue with the Roku.
I have switched out HDMI cables and have the issue with both new Vizio and Amazon Basics cables.
Last edited by davenelson
on Tue Jun 25, 2019 4:01 pm, edited 1 time in total.