Search engines such as Google maintain their own blacklists. Websites are added to this blacklist if a virus, malware or similar malicious software is loaded in the background when they are accessed. If Google also detects dangerous or illegal content on a website, this can lead to the website being put on the Google Blacklist.
If visitors have blacklist filtering activated in their browsers, a warning page, from the search engine, for example Google, will appear when they try to visit the website. The actual website (and the malware linked to it) are not loaded in this process.