If you use this direct link: https://web.microsoftstream.com/studio/meetings you will be able to log into your MS Stream area and it will list all of your meetings.
From the list of recording that then show find the videos you want for your Blackboard courses ‘Recorded Sessions’ area.
Next to the far right of the video title that you want, click on the three dots for more options.
On the new menu click on ‘Download Video’. It should start to download to your computer instantly.
Once you have the files you can follow this guide to upload it to Panopto: eLearn – Panopto upload