Assuming instance A and B are entering inserting same data (same PartitionKey/RowKey), then only one of the instance will be able to successfully enter the data. Other instance will get a 409 - Conflict
error. Can you not build your logic around that? In case an entity insert fails, you check for the error code and if the error code is conflict, then do something else.
I don't think blob lease would be helpful in this case. They are more suitable in scenario when you want only one instance to perform the insert.