To remove disk image from the blob repository:
1. Go to the Windows Azure Management Portal.
2.Select Virtual Machines.
3. Click on Disks.
4. Select the disk to be deleted.
5. Click on Delete Disk.
Now, the storage account can be deleted
Please note:
It is applicable even if you've already deleted all of your Virtual Machines and it shows 0; there still will be artefacts under the disks tab.
Disks are detached from a deleted VM asynchronously, it may take a few minutes after the VM is deleted for this field to clear up.