Hi lheath,
I understand you would like to change the left Navigation on your blog site, in order to show up the document libraries you created.
As far as I know, the blog site is a special site that has its own navigation showing up Categories list by default. Meanwhile, the navigation is not available to be changed by configurations in Site Settings. In addition, for users who want to show up the document libraries they created, they can achieve this by clicking All Site Content at the left navigation.
Thanks,
Maggie Li