To delete a message from a chat, hover your mouse over it and click on the trash can icon that pops up. The message will be instantly removed. Remember, if you want to change the content of the message you can click on the edit icon!
In its place will appear a system message explaining that the message was deleted.
Once deleted it will never be displayed anywhere in the app again. The actual content of the message that was deleted remains in the messages database for audit purposes. It is ultimately recoverable by organisation admins on request to Blink.