Hi Jack,

A container name can never contain a character like "/". This is also supported by the official documentation here. 

Therefor we can safely assume that the container is not available and can't therefor be deleted. Are you sure that this is container exists? Have you checked it with the Azure Storage Explorer?

So, let's assume that the Container name is "dev-test-filecontainer" and that ImageFolder is the first part of the blob name as there are no folders in Blob storage. If you want to remove this part of the Blob name, you will need to copy the blob to a new blob with the new name (thus without the "folder" part) and remove the incorrectly named blob. This because there is no rename capability in Azure Storage Blob (or at least there wasn't the last time I checked). 

I hope this helps you. Please feel free to ask more questions but make sure that you have the needed knowledge about Azure Storage Accounts first as this will likely solve all mysteries by itself.

Greetings,

Vincent