I often make passive-aggressive comments as a form of contempt. If someone is parked in a non-parking spot, I will say (just loudly enough that they probably can hear me), "I guess some people feel like they are entitled to just park where they like rather than in a spot like the rest of us." I only do it with strangers, though, not with friends or people I know. Well, maybe with our stinkie neighbour....
Anyway, passive-aggressive comments are often a form of contempt, indignation or self-righteousness. I suppose if someone were to hear me and respond, I'd like them to explain the rationale for parking where they did. So if you have to answer about a blog, maybe you could say "it sounds like you are upset that I have not read your blog yet." This way, you make the person take responsibility for their own feelings without giving in to the guilt trip they are trying to put on you. You don't have to answer to their self-pity. You do not owe them an explanation of why you didn't read it. But you can respond to their underlying issue and get them to own up to it.
Or if you want, you could go really deep and say "it sounds like you are resentful of the fact that I am too busy to read your blog as soon as it's posted." That will certainly get them to take a look at themselves, and let them know that your whole world does not revolve around them. It would be important to state this as matter of fact rather than expressing your own contempt over their passive-aggressive nature, though...