Apply for Blogger API v3 Access
Blogger's v3 API is a little bit more expansive than the v2 API, but doesn't allow open access yet. To get access to the API, you have to apply for it.
First, you'll need an API project.
Create a project in the Google Developers Console
Once you have a project, in the tabs on the left, select APIs under APIs and Auth. Search for the Blogger API, and click the OFF button to try turn it on. This will redirect you to form for API access.
Fill out the form!
The authorization process can take a while, and doesn't seem to be automated. If you find it's taking more than a few days, a few people have chased it up on the Blogger Developer Forum.
Happy API Developing!