When an SFTP user is deleted from the web admin portal, the SFTP user's files will still exist in Blob storage.
SFTP Gateway does not delete nor move (i.e. tombstone) the Blob storage path of the SFTP user's chroot directory. This is because SFTP Gateway leans heavily toward avoiding data loss situations.
If you need to clean up Blob objects after deleting the SFTP user, this needs to be done manually.
Also, Blob objects are not impacted when deleting a Folder from the web admin portal. Folders are just mappings or mount points. So if the SFTP Gateway Folder mapping is deleted, the underlying Blob objects will still remain.
In general, actions in the web admin portal do not delete files in Blob storage. File deletion in Blob occurs manually -- via the Azure portal, or via an SFTP client like FileZilla where the SFTP user has read/write access.