SQLVDI error when backing up to Azure Storage BLOB
Crikey, this is one of those "no idea how I fixed it" things.

Before I went to lunch, I could reliably generate the error in question by running the provided T-SQL, yet when I came back from lunch the BACKUP command completed fine!

The main thing I remember changing was on the Azure side, where I created a SAS (Shared Access Signature).

This is supposedly not required for SQL 2014 as it uses an actual Azure Credential to connect to the storage instead. I actually created the SAS for an instance of SQL Server 2016 that I want backing up to the same container, and that may have opened the access pathway to the container for SQL Server 2014 too!

Many thanks to Sean Gallardy for recommending ProcMon, which showed a whole ton more error log information than the "ErrorCode=(5)Access is denied" message did.