Here is a great question that was recently submitted:
“Many blogs allow DoFollow links with comments. Can these links ever hurt the Blog’s SEO? And if so, under what circumstances?”
Personally, I think the short answer to this is yes. If you have a trusted blog that is linking out to off-topic and low trusted websites with DoFollow, Google may start to query the external linking strategy and trustworthiness of the blog – thus affecting rankings as trust is one of the major factors, see this previous post.
I have long been a fan of a certain design blog which started getting spammed and it was clear that the blog comments/links were on auto approval – over time these spammy links got worse and I saw a drop in there rankings for blog posts which had been ranking well for a minimum of 12 months.
Moving forward, a lot of blogs now require a real name for comments so the whole blog commenting technique, in my opinion, may be loosing a little value in terms of getting good anchor text to assist rankings.
From a blog owner’s perspective, I always check the email address, web address and comments carefully. If I’m not sure about a comment, I will just remove the link or unapprove the comment.