Streaming Stick 4K shows a black screen with Dolby Vision content.

Exact same issue. Dolby Vision just stopped working across all platforms several weeks ago on my LG C7. I went into the advanced settings on the Roku and there's an option there to let the Roku change the refresh rate on your TV to presumably avoid this issue. I made that change, and now Dolby Vision is working on Netflix. I get a little popup saying something about the Roku changing the display rate, but otherwise it works. However - this did not fix the problem on Amazon, so I'm puzzled.