Question: When we do the blend and try the view the data in the viz, why does it only shows data from the primary data source?
Answer: A join combines two tables into a single larger table, so if you do a join and then look at the full data in the viz, you will see the table that combines the two (or more) tables that you have joined.
A blend doesn’t create a combined table. The Tableau documentation explains, “Unlike joins, data blending keeps the data sources separate and simply displays their information together…. Each data source is queried independently and the results are aggregated to the appropriate level then visualized together.”
In other words, with a join, the query combines the tables before Tableau aggregates the results in the view, whereas with a blend the aggregation happens before the results are combined.