I use tumblr to drive traffic and I rank on 1st page for over 50 keywords only from tumblr and few contextual links.
Use the link in the description of the blog. Also add 1 post 500+ words relevant to your topic with 1 link pointing to high authority site and 1 link to your site (use less exact match kw links and more branding, general words and nake links, including these links in the description). The link in the descriptions are fine without redirection but even the others, in the post , with redirections are still passing juice, so make sure u blast em with GSA after u set em with 100-200k spam links.
Also there is no need to register new tumblr blogs, just scrape some 30+ pa with the free TUMBLR HUNTER in case u dont have scrapebox.