Why is my screen black sometimes when opening?

Sometimes when I open my phone (or press the power button when it's already open, the screen doesn't show anything, just stays black. I know the touch still works, even when it's black, because I can hear the sounds - like swiping up to unlock the screen; or when I was just touching random places and hit the YouTube icon and a video started playing and I could hear the audio, but still no display.

If I get a call or text when it's like this, it will light up the screen, and function normally.

It's pretty random, but seems to be happening more frequently.