Since this is a BuddyPress forum, I’ll assume you’re talking about the Buddypress widget “Recent Blog Posts” and not the WordPress widget “Recent Posts”
You need to look in bp-blogs-widgets.php for the code. But, do not change the code within that file. It is a core BuddyPress file and any changes you make will be lost the next time you upgrade. Instead, make a copy of that file and create your own custom widget.