Blacklisting 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. Blacklisted 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 blacklisted.
Just long-hold any contact in the Textra conversation list screen > 3 dots top right > Blacklist..