Blocklisting is intended to hide messages from you that you're not
interested in, for example spam messages, people who are harassing
you that you want to ignore, etc. Blocklisted people don't
receive anything when they send you a text message, from their
point of view they wouldn't even know that they're blocklisted.
Just long-hold any contact in the Textra conversation list screen > 3 dots top right > Blocklist.
To un-blocklist a contact.
To remove them from the blocklist, go to the Settings page and select Blocklisted > tap on the 3 dots next to the contact you'd like > un-blocklist.
This contact and their messages will now show in your conversation list screen as expected.