Recently, I made an interesting discovery when making an attempt to import a Blackboard Discussion Board into Canvas.
When you export just the Blackboard Discussion Board using the Export-Package method. Canvas imports that package into the Announcements section of your course and not into the Discussion section of your Canvas course.
However, when you do a complete archive of your course and select only the Blackboard Discussion Board upon import. Canvas correctly imports the Blackboard Discussion Board into the Discussion section of your Canvas course.
Please refer to the attached PDF for a detailed explanation.