I don't think Blend will happen with 2 different date format. One is exact date and other is year quarter.
For data source 2 you can create a calculated field which convert date into year quarter format and then blend on it.
You can´t make a blend with fields totally different.
It´s necessary be the same.
thanks Tableau "suggest" (i assume) by placing a link next to these fields, just wanted to make sure