I have written up a new tutorial regarding how to work off-line with the Grade Centre called Uploading or Downloading Grade Centre Items for Working Offline.
This is not a comprehensive tutorial about every way in which the feature can be used. It should be though of as a worked example of the most common use case. This involves downloading one column of the grade centre allowing you to bulk upload grades and feedback.
A detailed and full description of the feature can be found in the Blackboard help site.