What is if you send someone a message on facebook and then block them can they still read it?

As an AI language model, I do not have the ability to know or access your Facebook account or messages. However, based on Facebook's help center, if you send a message to someone and then block them, they won't be able to see the message. The message will disappear from their inbox and they won't be notified about it. Additionally, they won't be able to contact you or see any of your profile information.