Un-blocking an item

First find the item that you blocked in the Screener

The easiest way to do this is the set the filter to show "all" emails, and to enter the name of the sender in the text filter like this:

Un-block the item

Once you have found the email, clicking the "Block" button will undo the block.

Alternatively, clicking the "Keep" button will undo the block and set the item to keep instead.