I'm configuring a hybrid image backup plan - monthly full, daily block - and I was intending to use Azure Archive tier for the cloud-storage, as any immediate restore would normally be available from local storage.
However, I am unsure of how the backup deals with the daily block changes when pushing to Azure and then to Archive tier, because obviously if this involves accessing the archive to update, then that will get expensive.
We do not need to read much data for backup purposes, except for retention needs (deletions).
You did not mention your retention settings for the image backup, but you should be aware that Azure Archive has a 180 day retention policy. So plan on keeping your backups there for close to 180 days in order to avoid early deletion charges.