There’s a tight correlation between Blogger and Blogspot, Blogger being the blogging platform owned by Google, Blogspot being a collection of sub-domains on the free domain providing service Blogspot.
Eventhough the blogs on the Blogspot domain are maintaining their traffic, the Blogging platform Blogger is struggling and have been loosing traffic and reach rapidly, in simple words, it seams that less people are blogging using Blogger and less people are signing up for the service.
Even Google seam to have ignored it’s platform, the Blogger Buzz: The Official Buzz from Blogger at Google have posted 2 posts only since December 2012, and both posts are in April , when the blogger freefall began.
Google may have been putting the blogger resources on Google+, but this month Google integrated the Google plus comments on Blogger / blogspot blogs.
What is really happening is not clear, because blogger blogs can be on blogspot or on independent domains, but blogger is the platform where the blogs are written, so if you’re to write a blog you need to go to blogger, blogger will redirect you to google accounts and then redirect you back to blogger.