This is a list of tumblr blocks and variables that are un-documented. Most were found in the new Optica default theme. You can find the official tumblr documentation here.
This is by no means comprehensive and is only from my personal experimentation. I take no responsibility for anything.
All that being said, this has taken me quite a few hours to put together. If you would like to donate to my paypal as a show of appreciation, it would be greatly appreciated. You can find my paypal here.
Examples are primarily taken from tumblr's Optica theme.