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Search engines can blacklist a website for so many reasons. For example If a website is involved in spamming or suspected of being used for hacking other websites, it will get blacklisted.
Also If a website has links to untrusted or malicious sites, it will get blacklisted and so many other reasons for websites blacklisting.
Use the Blacklist lookup tool to check if a website is blacklisted especially when the you notice there are no results for the website on search engines, then you should suspect a blacklisting.