well... have that person unblock you??? rather, check with them if they actually did block you ???
if they say that they didn't, then it's very likely some issue from carrier side.
one option other than to reach out to the carrier (or if they can't help) is to reproduce the issue and report it to samsung members app > get help > error report.
i suggest that if the other end of the conversation is also a samsung device user, then he/she do the exact same at the same time so that samsung can determine if it's your phone or the other person's phone (or even a network side) issue.