It does prevent blacklisted sites from being displayed in your search results, but that's it. It doesn't actually block them from your search results.

What's the difference? Simple. When you first search for a given term without blocking any sites, Google displays a list of 8 results. It increases when you scroll down due to the annoying infinite scroll feature, but the first set has 8 results.

However, when you blacklist one of those sites with this extension, instead of showing a list of 8 results without that site, it instead shows a list of 7 or fewer results, indicating that the site still exists in your search results, it's just not being displayed by your browser. This can sometimes result in strange gaps in displayed results if a page of search results happens to be entirely composed of blacklisted results.

2/5, does not function as advertised, but it does at least prevent blacklisted sites from being displayed.