Our blob storage account structure:
container name: simple
inside this container we have blobs:
As of today, it is not possible to do so at the folder level because as such there's no folder in Azure Blob Storage.
You can either create a SAS at a container level or at the blob level. If you want your users to only upload files with certain names, you can very well create a SAS with write permission on the blob name. Blob need not be present when you're creating a SAS.
Thus when your users upload a file using this SAS, it will be saved as a blob that you have chosen to name. So what you will do is create a SAS for a blob named
bb/ss.zip and give it to a user. When the user uploads the file, it will be saved as
bb/ss.zip in your blob container.