What do I need to consider when enlarging a basement apartment by opening up an attached garden shed?
A block wall that is actually part of the original outside wall of the main structure is most certainly a load bearing wall or part of the foundation.  It is possible to open a six foot section, but care must be taken to install a properly sized supported  header or if block is still going to be above the opening, a steel lentil. You will need some temp supports while building this new opening. A bit more details of your proposed project and some pics would be helpful.