A key to knowing what people are interested in reading about are the search terms people use to find a blog. But, things have been changing. Posted by Worldpress:
UPDATE: In September 2013 Google started to rapidly expand the number of searches that it encrypts, which results in a higher proportion of “Unknown search terms” in your stats. According to some sources, this expansion will eventually result in encryption of all Google searches. This is being done for privacy reasons by Google when someone searches at Google.com, before a visitor arrives at your WordPress.com site. Therefore we don’t have any way to unhide the search terms. We recognize this means a loss of stats information for you and we will look for other ways to show you how users arrived at your site.