Following changes to Blackboard, this process no longer works.
Having copied your Blackboard link to the clipboard and made your course accessible to Guests. The link will only work for users who do not have University accounts or for University users who are not logged into Blackboard.
Open this page https://mle.southampton.ac.uk/tools/blackboard/guestdeeplink.html
Paste the link into the textbox on the above linked page and click the button. You will be provided with a short go.soton link.