I blocked someone from contacting me on iPhone. I then sent an angry message to them that I regret. Did they see the message?
Blocking someone and being able to send them messages are two different things:


  Yes, you have only blocked seeing that they called or texted you. Blocking has nothing to do with outgoing calls or texts.


Your iMessage has most likely been delivered to, and displayed on, one or more of the recipient’s iOS and macOS devices.