Add a track to a special playlist in Spotify, it'll post to your Tumblr blog
If
New track added to a playlist
This Trigger fires every time a new track is added to a playlist you specify.
Then
Create an audio post from URL
This Action will create an audio post on your Tumblr blog from the given URL to an MP3 file.
Discover more time saving integrations for Spotify and Tumblr (legacy)