To change Blog Excerpt font in Squarespace, you can follow these.
#1. Determine your Blog Page layout
Edit BLog Page > Edit Section
At Layout, your Blog page layout will be shown
In my example, the Blog page layout is Single Column
#2. Click on the Site Styles icon in the top right of the Header
#3. Choose FONTS
#4. Next, choose Assign Styles
#5. Scroll down to Blog: Single Column – Excerpt , and click on this option
