When to use data blending and joining?
It is generally preferable to avoid data blending when you can combine the two data sources outside of Tableau. If this is not an option, then you must identify at least one common variable shared by the two data sources you want to blend together. When possible, go for a join rather than a blend. If you need to combine two data sources and for whatever reason cannot manage to join the data outside of Tableau, your only option is a data blend.
A simple example is having (a) a data source with three columns including location names and latitude/longitude values, and (b) a data source with location names and detailed information about each location. You could build a map using (a) and then blend in extra supplemental information using (b), where a relationship is built by connecting the data sources based on the location names.
When to use joining:
You can only use joining when your data comes form the same underlying source (for example, the same Excel file or Access file).