I blocked out some things I didn't mean to. Blocking an ad also blocked out graphics that I wanted to see (on IMDB).

Does anyone know how to reset Ad Block Plus to its default state?

AdBlock Support may be your best bet, but here's what I'd do in v 2.1.5 for Safari:

1. Click on AB's icon.
2. Click on "Blacklist".
3. Click on "Edit".
4. Scroll down to the bottom of the list and delete what will likely be your last, i.e. most recent, entry.
5. Delete it.
6. Click on "Save".

Things have changed in recent versions, so I don't know if those steps will work with the version you're running.