If a blogger posts something negative about your company, don’t ask them to take down the post. It does not work that way.
If their post is factually incorrect, ask for a correction. If it is a matter of opinion, there isn’t much you can do about the post itself.
The best thing to do in such situation is to bury it with positive coverage. Pick out a positive story about your company that would appeal to bloggers (this means you will have to read the blogs relevant to your industry). Pitch the story until you have a few blogs writing about it. Then hype those blog posts around blogosphere, so your friendly bloggers will have some inbound links for their efforts. In this way you can insure search results will be positive for your client.