Microsoft has added a new Append Blob as part of its Azure Storage. The Append Blob is designed for scenarios where you need to efficiently and continually append data to the end of a file, e.g. audit logs, event logs, etc. When an Append Blob is appended, the new data is automatically committed and is immediately available.
Append Blob requires the latest Azure client libraries, Power Shell cmdlets and/or REST APIs to access. Pricing isn’t specifically mentioned on the Azure web site yet.