The easiest way to do this is by rules and by using the mail action.
The rule can be set through the Repository viewer on e.g.:company home/sites/swsdp/blog The first one is to set on incoming content and the second one is a bit harder.
There is a fm:commentCount which gets updated and you can check on that one on a change rule. This can be done by a behaviour to check if the property has been changed or you can go through the object and locate if a new comment has been added (check by date for example)