What does the official Tumblr app do? Two things. Two fascinating things.
1. It goes to Tumblr and only to Tumblr. You can even make it full screen, so that your computer displays 0% non-Tumblr content.
2. It lets you post to Tumblr from almost anywhere on your Mac. If a window has a share button, you can share things from that window to Tumblr.