How to confront possible theft of ideas?
I found myself in a similar situation once, also with a blog post, though in my case the poster fully admitted posting my ideas and appeared to see nothing wrong with it.  Fortunately, I later determined the specific idea wasn't as fruitful a line of inquiry as I initially imagined and happily moved on.

So, my suggestions to you:


It is fishy, and one clear lesson to learn is to not trust this person with your work in the future.
Hold on to records of your communication, in particular any emails where you discuss this idea and/or send him drafts.  As was pointed out in the comments, blog posts aren't publications, but it's still possible someone notices in the future and asks you about it.  Having the email where you sent your work to the poster for review from before they posted it is a solid defense.
Mention it to your advisor, but I would personally only do it as an aside, as something that annoyed you.  You can handle it!  If they, knowing you and the situation and the work itself, have other suggestions, they'll offer it at this point.
Don't bother bringing it up with that individual.  If they did steal the idea from you, they can just deny it and all you have is circumstantial evidence.  On the other hand, if they had indeed done similar work on their own already, then it seems they posted this in order to get their name on it before you (they can always claim their anonymous blog later), in which case you're competing to get this published.  Therefore,
Take it as motivation to push through and finish your work both well and promptly.


Good luck!