Hello a_m377,
Welcome to Apple Support Communities!
We understand you're attempting to block someone on Apple Music and the app crashes. We'll be glad to assist.
Make sure your iPhone is updated to the latest iOS version. If there is an update available, make sure to back up first:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
Updating will help resolve issues with apps crashing and provide the latest update for the Apple Music app.
When this is complete, try blocking the person again.
If you experience the same thing, please complete each step here: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
Take care.