After you have found out the category id, just go to Dashboard -> Appearance -> Customize -> Theme Options -> Blog Options -> Exclude Categories in Blog and add the category id on the text field.
I should have said more, sorry, I did tag them per category with the testimonials category. Now they do show on the testimony front page widget (great!), but – even after I “excluded” that category in the blog options, they are still showing on the blog page (weird). The blog options to exclude is not working.