You can access Blackboard sites for modules you have completed in previous years of your course through the 'Blackboard Sites' channel. This channel contains two tabs: Current Sites (which has modules you are currently enrolled on) and All Sites (which lists all Blackboard sites you are enrolled on by academic year). If you click on All Sites you can access any module site from previous years.
You can customise which sites are listed in Current Sites by clicking on the chevron next to the site name, and choosing 'List in Current Sites' or 'Hide from Current Sites'. If you think a site may be missing from your channel, remember to check in the All Sites tab.