@CristinaDeMeo Importing multiple Blackboard courses into one Canvas course will not result in content being overwritten (though it may result in duplicate content if the Blackboard courses contain same items). Content is overwritten during an import in Canvas only when the same items are imported to the same course from the same original course. If you are importing Blackboard course exports, Canvas will NOT identify them as the same source even though they are the same source type, and content will not be overwritten.
I do agree with @paul_fynn that you may want to use separate courses in Canvas to import the content and then merge the content into one Canvas course by copying the content from the other courses in Canvas. Though most content imports from Blackboard without issues, that is not always the case, and you may want to review the imported content and make needed modifications and updates before using in a Canvas course with students.