If you are seeing a 'Blacklist' rule or error message:
This is usually when the person you are trying to reach has send a text message to their carrier to block some numbers trying to reach them. If someone sent you a text message 'STOP' then you will be prevented from sending messages to that person.
If you know the person, you can tell them to text to your number with the message 'START' which should usually unblock.
The other party will also need to send a text message to you with a message 'START', 'YES' or 'UNSTOP' to continue receiving messages from you. Please note that this is a policy implemented by many SMS carriers in the US. There is nothing we can do as this is between the person you are trying to reach and their carrier.