thanos-io/objstore#35 introduced a change to the way Azure endpoint default value is calculated. Previously, setting endpoint: "" in the object storage config would mean the default endpoint would be used. In v0.31.0, setting endpoint: "" overrides the default, leading to errors when attempting to create a bucket client.

The issue was introduced by f9d07a3 which in-turn brought in the changes from thanos-io/objstore#35.

This issue was first discussed in a slack thread.