When you send an e-mail to your students from within a Blackboard course, the e-mail is sent to the e-mail address that is in the student's profile. While the e-mail is sent from Blackboard, it comes from the e-mail address that is in your profile.
When you send a message to your students, the message is sent from your Blackboard account, to their Blackboard accounts. They must log in to Blackboard to read the message.
An announcement is not automatically sent to a student in the way that an e-mail or message is sent. As an option, you can choose to have the system send an announcement to the recipients via e-mail. Announcements appear on the student's home page, in a section for announcements. Of course, the student must log in to see them. You can set a time limit for an announcement. That is, you can tell it to appear and disappear on specific dates and at specific times.
Alerts are automatically generated by Blackboard...