I have the same problem. I have the Surface Pro 7 (256GB SSD, 16GB RAM, i7) and this problem seems to be happening more often than before.
When I am doing my regular work on Google Chrome or Microsoft Word, the screen randomly turns black. I find that the screen didn't turn off, I looked closely and found that the stuff on the screen was still visible, meaning the LED backlight must have turned
off for no reason. I don't know if this is a hardware or driver problem, but I can close the type cover and open it again 5 seconds later and the screen returns to normal. It is really annoying when it happens, sometimes it just flickers on and off for a short
period of time, before turning black.
Oh, and this happens both when plugged in and on battery and happens especially often on Google Chrome (maybe it's because I use that the most).