Blacklisted malicious website/content is only one reason for blacklisting, other reasons could be phishing, spam (SEO and other), scam, untrusted certification, other protocol issues, defacement. So when for what reason or other a site is not to be trusted and it is better not to visit such a site it can come blacklisted. AV can block for that reason, various extensions can block for such reasons and even ad-blocker extensions can block.
When a site is blacklisted because it could infest it also is blocked, so you cannot visit it to get an eventual infection etc.
AV have their own blacklist, so there is a Bitdefender TrafficLight blacklist, there is the DrWeb URL checker malicious websites list,
and there is a blacklist for bad web reputation from WOT.
Blacklists can be looked upon as traffic road signs as where not to go on the Interwebs. When you haven't gone out to some destionation and gonna visit there for the first time checking extensions like Avast Online Security, DrWeb's etc. will give you an indication - green (secure as far as known), yellow (visit with taking care), red (do not venture out there).
When you wanna download something from the Internet I would like to check it here first:
https://www.virustotal.com/en/By the way when you want to run a program your Avast will scan it before giving it to run, better safe than sorry!
polonus
