Just a note...
There is an open-source github project called "blobfuse" which uses the FUSE libraries to write to Azure Blob storage. I'm not including a link because I don't think unsupported mechanisms are appropriate for backups, but you can google it yourself if you wish?
Following are better options...
One presently available option for less-expensive storage for RMAN backups is "Azure Files" which on Linux VMs are mounted using the CIFS protocol (a.k.a. SMB or Samba). I recommend using the "premium" level of service of Azure Files, and I also recommend specifying the additional mount.cifs option "cache=none" ( https://docs.microsoft.com/en-us/azure/storage/files/storage-how-to-use-files-linux
Azure Files premium over NFS v4.1 starts in "public preview" today (15-Sep 2020), so that is another option. This offers the use of the NFS protocol (instead of CIFS) for remote Azure Files storage, for the premium option only ( https://docs.microsoft.com/en-us/azure/storage/files/storage-files-how-to-create-nfs-shares?tabs=azure-portal
Last and best, the ability to mount inexpensive and fast Azure Blob storage directly over NFS v3.0 started in "public preview" in July, available in the East US, Central US, and Central Canada regions. This will allow data from the DEVICE TYPE DISK adapter of RMAN to stream directly to Azure blobs. ( https://azure.microsoft.com/en-us/blog/nfs-30-support-for-azure-blob-storage-is-now-in-preview/
). This option is expected to become GA in Q1 CY2021.
Hope this helps!