Blocking a non AT&T iPhone User

If the person is not using AT&T, they will not receive any feedback stating that their message was blocked. 

Here's an article with more information: http://www.att.com/esupport/article.jsp?sid=KB92823#fbid=Z6SDS7kpLnk

Specific to your question:

Blocked Text Messages

Text messages will be blocked and AT&T subscribers will receive the auto-reply text message: The wireless subscriber you are trying to reach is not accepting incoming messages at this time if:

• The number is on the Smart Limits subscriber's blocked list.
• The Smart Limits subscriber has reached the text message limit.
• The text message was sent to the Smart Limits subscriber during a Time of Day restriction period. Note: Text messages are not delivered when the time restriction is lifted.

If the sending number is not an AT&T subscriber, they will not be notified that the text message was not delivered.