Can I send a message after I nail the blacklist?

I can't. According to the query, official website knows. If you are blacklisted, you cannot successfully send messages to the other party. You will be prompted that you are not the other person's friend. This is a function that is set to protect the privacy and safety of users. After joining the blacklist, you will not be able to communicate with this user in any form, including sending messages, voice calls, video calls, etc. At the same time, users can't send any information.