There's no way to explicitly respond to a particular comment, the comments are also not threaded. All you can do is post a comment yourself and maybe '@' the commenter you're responding to so (s)he knows you're responding to their comment. If you need a more powerful solution for blog comments you might want to look into something like Disqus.
Just following on from what @ThomasBorowski said, it is true that you will need to go into your own blog post as if you were a customer in order to be able to respond - as of yet we don't have a built in way to do this through the admin.
You mentioned that you would like to 'personally' respond, it is important that you sign off on your replies - in the sign off you could include your name, and then the name of the store following to make it look more official.
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