This is the best spoiler filter I have found so far. It allows me to walk away from a page if it has stuff I don't want to see.
There are ridiculously few add ons for this...
But it could be better:
1: give us an option for the level of transparency on the block screen.
2: this add on only scans the page when first loaded (as far as I can tell), so pages that load more content continuously can slip through. Can you make the scan repeat? This can be a real problem on youtube or facebook
3: I think the keywords are case sensitive, which is why I have about 6 versions of the same titles on my list. There should be an option to make it not case sensitive. (or at least tell me that it isn't)

this addon is useful but:
- the transparency is too high, you can read too easily what is behind, there should be a setting for transparency with "completely hidden" as default
- sometimes is too slow, you can read a text or see a picture before it gets hidden

The addon still needs some work, I found 2 major flaws:
1) the panel that covers the page to prevent spoilers is too transparent. You can see images and text through it very easily and if spoilers are on the upper part of the page, you're doomed
2) the addon sometimes doesn't work properly. I was trying to avoid "masterchef" spoilers. Entered www.gazzetta.it and no panel appeared, however searching the word "masterchef" on the page (a friend helped with this :D) quickly revealed that there was an article about it. Sometimes the panel appears only after you click on the page.