When you use Data Blending and row-level (non-aggregated) calculation from secondary data source, you need to add the granularity which identify each row in the view.
But in your case, it is not possible to add Order Date because primary data source does not have it.
So if you want to separate colors by today's month or not, I recommend you to separate aggregation and use Measure Names & Measure Values.
1. Create 2 calculated fields on secondary data source.
2. Add both field into the view with Measure Values, and set Running Total on both fields.
3. Add Measure Names to Color on Measure Values' axis.
(4. Change color allocation and title of color legend as you like.)
Please see attached workbook.