I believe I understanding the desired result but would like to clarify some termninology that is being used. First the difference between joins and blends are outlined here: what is the differences between Data Blending and Joining the Tables?
Also, currently the ability to join to published data sources is not yet built into the product. Please vote for the following idea to show support for this feature to be added to the product.
Can you try and reconnect to the underlying data of the published data source reublishing after creating the join?
Hope this helps!