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You may want to create a sign-up sheet for Office Hours in your Blackboard course.
Here's how to do it using Google Calendar in a way that is convenient for students to use.
Video link: http://screencast.com/t/40XGIo9egj
Note: To sign up for an office hours slot, students must have a Google Account (this is how the Office Hours calendar knows the names of the students in the slots without having to have them explicitly type their names). All students at Northeastern University already have Google Accounts via the G-Apps service (more information on G-Apps is here).