It seems that some of the ads that BlogCatalog has been carrying are for websites which infect vulnerable computers with malware/ trojans.
Here is the Google report on why they are issuing the warning.
It seems that malware distributing flash banners associated with the IP address 188.8.131.52 have been known about since at least October 2008 http://msmvps.com/blogs/spywaresucks/archive/2008/10/31/1652595.aspx
Norton Safe Web reports 18 virus threats at that IP. https://safeweb.norton.com/report/show?name=184.108.40.206
Ok you we can say that BlogCatalog has been carrying the ads in good faith and how were they to know that these adverts were disreputable.
Well you can just about get away with generous line of thought until you scrutinise some of the ads BC has been carrying a bit more closely. 7 days ago Polybore started a discussion complaining about the low quality of the ads on BC.
During the disscusion Polybore scrutinised one advert that was particularly bugging because it was advertising link selling which Google really frowns upon. In a sense buy a link from us and watch your Google Page Rank fall like a stone which looks like a scam. They imply they can get you a link on the PR 10 US.gov website. Yeah right and Polybore can get you a nice condominium on the moon.
The ad rotates to display how much it costs to buy a link, starting from $1.48. Now the fact the site advertised is selling links is suspicious in the first place, however it gets worse. Polybore visited the site (fortunately without getting a Trojan) that is selling links to find it has a Google PR of 0, yes zero. Who is going to buy a link from a site with a PR of zero.
Polybore has to say that BC has brought this problem on themselves. It was obvious that these ads were of a very low quality and some were for products verging on scam. BC could have avoided this problem and, unless BC gets their ads sorted, it is likely to happen again. It can take a while for these warnings to be taken down by Google etc. and it is just not worth the risk.
Come on BC get the ads sorted.