Which page you're on? If its the home page (https://www.pixiv.net/), then yeah I haven't added support for it yet.
It should work on artwork pages (https://www.pixiv.net/artworks/<'ID>) and tag/search pages (https://www.pixiv.net/en/tags/<'tag>)
If it doesn't work on those, or there is some other issue, you can post a report on Github.
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Update: added support for blocking on Home page, and partial support (some image shelves are blocked, some aren't) for Illustrations and Manga pages. Full support will be added some time later.