Just like the comics, destruction on a planetary scale. Hell, multiple planets without his helmet's fork used to focus his energy.

He doesn't seem to be as powerful as his comic book counterpart. It took him several seconds to kill 838 Strange at what sounded like a normal volume voice. I think his scream would still be plenty powerful, but not on the level of comic Black Bolt.

Yeah, even in the show, it's just the scream and he doesn't have his other abilities. It is slower than his comic counterpart. Though pretty sure a full on scream can still destroy a planet based on the whisper's range.

We never heard him scream in the MCU. In Inhumans we see him use his powers 3 times.

1. Shortly after teriginese, when his powers are not yet fully developed, he accidentally kills his parents. (Normal volume)

2. A small sound escapes him as he is being beaten up. This throws a car through the air.

3. He speaks at a normal volume, causing the palace to collapse.