From the wordpress.com Stats support page:
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.
Any blog owner would love to see the search terms leading to his blog, but I damn respect Google’s decision. Been using Google’s encrypted version for some time, but 99% of the people don’t know of its existence. And it seems now you don’t need to anymore.