Text message vs iMessage on iPhones
No: blocking will actually generally result in your phone sending as an iMessage that doesn't get delivered. Any number of things could still be happening to send as text. You or the other person may have sending/receiving as SMS enabled or iMessage disabled. You or the other person may not have a proper internet connection for a number of reasons. iMessage servers may be down–which, although incredibly rare and usually noticeable, has happened on a few occasions when AWS servers are down or other problems occur.