Q: What do you do if someone blogs bad news about your company?
A: Say thank you.
Huh?
It's natural to get defensive when you get a negative online post. But take a moment and instead of reiterating your company policy and explaining your procedures, say "thank you".
After that have a conversation with that unhappy customer, but have it over the phone.
Listen to their story, empathize and offer to make it right. In fact, sweeten the offering so much that you can go back later and ask them to take down their post or at least repost a positive.
A negative post is an opportunity to shine. Respond don't react.