"Specifies the blob directory that contains the blob files to be downloaded. The blob directory is a virtual hierarchical structure."
This would be the directory in your container that has your files, if your file is in the root of the container - then leave it blank.
This google image search, reveals some examples: https://www.google.com/search?q=Azure+blob+download+task+in+SSIS+BlobDirectory&sxsrf=ALiCzsa8EJ-RFy4v8pvU_xVJfLuZgndybw:1663881478475&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjW7ePDqan6AhXO5HMBHXxWCoUQ_AUoAnoECAEQBA#imgrc=IEfSaFnSl_A9DM