You can use these Tumblr features as Feeds content sources:
Username: Enter the username of a tumblr blog (found in the tumblr url) as the source. For example, http://generalelectric.tumblr.com you would enter
generalelectric into Feeds to pull in that blog's posts.
When you add a specific Tumblr account, the account must be public. If you find that you cannot add the posts to Feeds, check that no privacy settings are enabled.
Hashtag: Pull in any posts that have a certain hashtag on Tumblr. More information about inserting tags into a Tumblr post here.