It has been reported that Google treats ESL-blog as a URL with the keywords ESL and blog. ESL_blog would be one keyword, literally ESL_blog (I can't imagine anyone ever searching for that). I've written about domain names before, and they are not a huge factor but if you believe as I do that every little bit helps, use dashes not underscores in your domain names, at least for now. Google being as smart as they are should correct this seemingly dumb distinction between underscores and dashes soon (I hope).
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