A blob is a record that contains the metadata about a file and a key for where that file resides on the service. Blobs can be created in two ways:
Subsequent to the file being uploaded server-side to the service via
Ahead of the file being directly uploaded client-side to the service via
Blobs are intended to be immutable in as-so-far as their reference to a specific file goes. You're allowed to update a blob's metadata on a subsequent pass, but you should not update the key or change the uploaded file. If you need to create a derivative or otherwise change the blob, simply create a new blob and purge the old one.