Sites are blocklisted when authorities — such as Google, Bing, Norton Safe Web, McAfee SiteAdvisor, etc. — find irregularities on a website that they believe to be malware. Malware can come in many forms: trojan horses, phishing schemes, pharma hacks, email, or information scraping. Most often, the website owner is not even aware that they have been hacked.
However, it’s in the search engine’s best interest not to show infected results, as they don’t want it to damage their integrity. There are several different categories for blocklisting, depending on why the website was blocklisted. For example, some websites are blocklisted for having spam, others for having phishing links, or more generically for having malware. We will dig deeper on types of blocklisting reasons below.