Currently the ability to filter a secondary data source to show only relevant values is not built into the product.
If you'd like to vote for this feature request, there's a community post here: https://community.tableau.com/ideas/2747#comment-23291
You may also find some ideas on workarounds within that thread.
Hope this helps!
Use Left or Inner Join for your datasources instead of blending to get this option.
Thanks Cherly for the reply..
The reason you are not seeing an option to select only relevant values is because that option is automatically chosen for dimension filters on secondary data . Think of secondary data as the right table in left outer join, where only matching data (based on the linking field) is used. This is the feature with dimensions. For filtering on measures however , you can choose between 'Only relevant values' and 'All values in database'. So in the example below, count of date will have both the options, but date as 'Individual date' or 'Relative Date' will not.
Hope this help.