Update / Answer: The single blue arrow next to Google search results is used for keyboard navigation. Try moving your arrow keys up and down. Thanks Ahmad P. for the answer. For more info read the comments at the bottom of this post.
Google must be up to their testing again. I thought my eyes were playing a trick at me at first, then thought maybe I had installed a new plugin in Firefox and that was causing it. But after testing several different search terms with several different browsers including Firefox, Internet Explorer and Chrome; Google was indeed showing a single blue arrow there next to one of the search results on each page for nearly every search query I tried.
Which result the arrow pointed at did seem to be somewhat random, but I’m sure nothing Google does is random. For instance one time I searched for “SEO Blog” and it showed a blue arrow pointing at the second listing which was SEOmoz.org. A bit later I tried and the same search with a different browser and it showed the arrow next to the first listing which was Wolf-Howl.com.
I searched all over the web and couldn’t find any information about the mysterious Google blue arrow typically found next to the first result. Perhaps we’ll learn something about it tomorrow…