Jan 14, 2015
Feb. 20, 2015 Update: Tumblr added an Embed button to individual blog post pages. Get the Embed Code from a Tumblr Post URL:
If it's not a post on your own primary blog, make sure that you are following the blog to get the embed code from a post URL.
Note the post ID number in the post's URL: http://www.davidslog.com/98260713170
Now increase the post ID's last digit by one: 98260713171
Go to the Desktop Dashboard at https://www.tumblr.com/dashboard then add /2/98260713171 to the end of the web address and go to the new URL.
The post appears in your Desktop Dashboard.
Click the three-dots/ellipses icon on the lower right of the post and select Embed for the embed code.