Blocked numbers

What do you see if you send them a message. Blocking someone's number does not block your ability to send them a message, it only blocks your receiving a message from them. Since that blocking is on Apple's end, it may take a period of time after unblocking for you to be able to receive a message again. See if any of the information in this support document helps you fix your issue. Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch - Apple Support

I suggest waiting 24 hours and see what happens. From there, if you are still experiencing a problem, delete any conversations the two of you have on both yours and their devices and then start a new conversation and see if that works.