Am I blocked on Messenger?

It's difficult to say for sure, but given the details you gave, it's conceivable that the Messenger user you're attempting to message has either banned you or is not responding to your messages. Your message's transition from "Sent" to "Delivered" without being marked as "Seen" signals that the recipient may not be actively responding to your communications.

But it's also important to take other options into account. People occasionally could have Messenger settings set up so that it doesn't display the "Seen" status or that it doesn't send alerts for your discussion. Another possibility is that the delay is being caused by technical difficulties on either your end or theirs.

If your discussions with other people are functioning normally It would be worthwhile to try contacting the individual by an other mode of communication, such as a phone call or email, to see whether they reply. You will be able to comprehend the issue more clearly in this manner.