MATLAB Grader is integrated within Blackboard as a content/object type of assignment. Within Blackboard you can group MATLAB Grader problems within folders, and then set conditional release rules based on times and dates to control access to the folder for groups of students. Additionally, a group of MATLAB Grader problems can be added to a summary column in the Blackboard gradebook which would treat the set of problems as a single "test" for scoring purposes. This is how to restrict access to a set of MATLAB Grader problems during a specific time window.
This is achieved using an online proctoring solution or locked down browser, which can be organized by the IT staff that supports the instructor's Blackboard instance to pursue an online proctoring solution.