I’m sorry that I didn’t make myself more clear. Useful comments are a prerequisite but not sufficient by themselves. If you post a comment that really flatters the blog owner, his company, and his product or service, it makes it much harder to not approve it. If your competitor was saying really great things about your company wouldn’t you want those comments widely publicized?
You can’t just promote your own company on your competitor’s blog, you must help your competitor promote his company if you want to get your comment approved. I know that may sound counterintuitive and it actually is, but hey, whatever works.