The most obvious sign that someone blocked you on iMessage is the Delivery Status. You’ll see a Delivered or Read option whenever you send an iMessage.
You can’t block someone on iMessage without blocking them from calling too. If you still suspect your friend blocked your iMessages try giving them a call.
If you tried calling and texting but received the “This number is unavailable” response, we need to determine if the phone has no service or if you’ve been blocked. The best way to check is to call from another unrelated phone number.
One way to know for sure if your friend meant to block you on iMessage is by checking your social media accounts. Although it may sound odd initially, most people will block others on all platforms, not just iMessage.