I thought I’d post this in case anyone else has run into this issue (I know there are other baseball fans in the group!).
On my Apple TV 4K, I have my HomePod set up as the speaker. When I tried to start any game in the MLB app, I suddenly had a black screen and no sound.
This thread explains the fix and provides a theory to explain the behavior.
TLDR: Set your audio output to the TV speakers, launch the game, and then set the audio output back to the HomePod.
Go Cardinals! 
…
Ok by me if the Cards do well. They’re in the NL after all, so my beloved Twins don’t see much of them. 
Oh, and if you want to see a game that’s good so far, check out Minnesota-Milwaukee. It’s the top of the 5th and both pitchers are perfect so far, with eight strikeouts each.
Can’t be as good as the fight that broke out between the Reds vs Cardinals. 🥷
I managed to miss that.
And I shouldn’t have mentioned the perfect game, as one of the pitchers just hit a batter.
Fortunately it was Milwaukee’s pitcher.