Comment by monsieurbanana

The blacklisted URL in this case is found in a downloaded file from a S3 bucket.

Other people downloading the same file would get the same "protection", but in this case this goes a step further:

The S3 bucket itself gets then blacklisted. As it was a private bucket, one of the ways this could happen is that once chrome found the blacklisted URL, it sent back to Google the url (s3 bucket) where the file with the blacklisted URL was found.