The following screenshot shows a simple example of data blending in action:
There are two data source connections defined in this workbook, one for the Superstore data and the other for Superstore Sales Goals. The Superstore data source is the primary data source in this view (indicated by the blue check mark) and Superstore Sales Goals is the secondary source (indicated by the orange checkmark). Active fields in the view, which are from the secondary data source, are also indicated with an orange checkmark icon.
The Sales measure has been used from the primary source and the Goals from the secondary sources. In both cases, the value is aggregated. The State dimension is an active linking field, indicated by the complete orange link icon next to the field in the data pane. Both measures are being aggregated at the level of State (Sales by State in Superstore Sales and Goals by State in Superstore Sales Goals) and then matched by Tableau based on the value of the linking field State.
Data blending will be done based on an exact match of the dimension values for the linking field(s). Be careful as this can lead to some matches being missed. You'll note that the indicator in the lower right of the previously viewed which indicates > 1 null (at least one null value).
An examination of the data reveals that the state value in Superstore Sales is District of Columbia, while it is DC in the Superstore Sales Goals data source. You'll either need to fix the values in the data source, create a calculated field to change values, or change the alias of the value in one data source to match the value in another. For example, you could right-click on District of Columbia in the row header, select Edit alias... and set the value to DC.