Yarr, me hearties!
An issue with Avast (an Anti-virus popular for personal use as it's free), led to many websites having the black mark placed upon them erroneously.
The error only affected computers that use Avast for their virus protection.
Yesterday, when a user updated their PC with Avast’s latest Virus Definitions, and then attempted to surf the sea of iniquity that is the web, Avast identified many sites as being infected with malware, virus's and trojans, despite no such threat existing.
Many innocent sites were blocked by this faulty update, sites including Finextra and Youtube.
Avast have since issued a blog to confirm that there were in fact no sea monsters, just, "an error that resulted in a good number of innocent sites being flagged as infected".
Here is the blog from Avast explaining the Update error.