Blocked individuals who call a customer will hear the message:
"Welcome to Verizon Wireless. The number you were trying to reach has calling restrictions which has prevented the completion of your call."
If someone tries to text you from a number you have blocked they will get an error that the text failed to send. You will not receive any notification that a blocked number attempted to text you.
Are you sure they get an error that the text failed to send? I've been told that it looks like it went through on their end but obviously, you (who blocked them) just doesn't get it.