Watch this video from Google to see what Google specifically acknowledges is spam and may affect your organic placement.
Nuggets from this video:
- Blog and forum comment spam can impact your placement on Google.
- Keep an eye on your comments look for small comments with links and links in a signature block.
- I recommend on your blog only allowing comments to be posted that you approve.
- It is not unusual for a blog to get over 1,000 spam comments a month. If you are set to auto approve all comments this could negatively impact your placement on Google.