I used this device to clear a friends "pyramid".
http://www.homedepot.com/p/BrassCraft-Small-Rubber-Drain-Bladder-BC00331/100621876You feed it into the toilet drain and it expands to the width of the pipe and then sprays a hard sharp spray below. If you get NO outflow you are going to need to stop as the tank will fill quickly.
If you get no love there I would use one of the clear drain/flush attachments that allow you to run water up into your tank from the drain side. If you can get some flow this way you can return to the top side and get that water jet eating away at the blockage. You may have to go back and forth a couple of times but eventually you will get things flowing again. Once you do, fill the tank about 1/2 way and add something like some Pine-Sol to kill the odor (no chemicals will help, you just need time and water). Use the toilet as normal with the black valve closed until at least 3/4 full and dump. Fill the tank about 1/2 way and use as normal again. Repeat process but never, ever, again leave your black valve open!
There are services available that can do a tank flush professionally and restore your tanks to nearly the same condition as when they were installed. I recently had this done just to get rid of the crust that accumulates over the years on the sides of the tank. It cost less than $200 and it's likely I will not need to have it done again for another 10-12 years.
This is the service I used.
http://www.allprowaterflow.com/news.asp?id=4