Once you have designed your rubric, you can set it up in the Rubric tool which can be accessed from the Gradebook. You can then add the Rubric to your assessment and use it to mark the submissions.
Below is a recorded demonstration showing how to add your Rubric to Blackboard; link it with an assignment; and use it to mark submissions to the assignment in the Gradebook:
Creating a Rubric in Blackboard to mark assignment submissions
Further guidance on using Rubrics is available from the Blackboard webpages.
Please also see the following recorded presentations provided by an academic in the College of Business and Law, which give guidance and techniques around designing your rubric:
Some further recordings of practical examples and advice are also available.