BuddyPress.org

Have a look at the recent sitewide posts widget – the code in there should give you the pointers you need.

You could also look at the functions in bp-blogs-templatetags.php

Or, I suspect best is simply to filter the activity stream for blog posts, and use the ids embedded in there: https://codex.buddypress.org/developer-docs/custom-buddypress-loops/the-activity-stream-loop/