Not only does Google have a tool now which pretty quickly spots spam posts and prevents the comments from showing at all, there's this little thing called a "nofollow" tag which I personally can use if I decide that I don't want a link from my blog to be followed by a Search Engine robot. Like the one I used above with my 'clueless dentists' link. Get the picture?
Oh, and in case it wasn't already obvious, I work in this space professionally and can spot a purchased/promoted link a mile away. You would be better served by spamming someone else's blog-or rather, not doing it at all and going about link building the old-fashioned way-by asking for the link.