I can't figure this out either. A friend actually blogs on tumblr, and I find the lack of obvious comments really irritating (even after she said she turned comments on). I guess it's the design, per the Ars article.

* I'm a developer and not afraid to get my hands dirty but I'd rather have something self hosted because I'm not interested a potential cracking vulnerability I'd get hosting my own.*

As others have said, Wordpress.