I realize that you probably can't really read the dates on that graph, but that is take from Google Analytics, for my primary blog My Quality Day, showing the past year. The graph is actually of the number of hits per visit, not visits, because the drop is much more obvious on that metric. But you can clearly see that there was a significant and permanent drop during the past year.
And what is the date around which that phenomenon occurred? March 2011... What else happened then? That was right after Google changed their algorithm. Interesting coincidence, or does it really mean something? I sure can't say with any kind of certainty.
Google's own spreadsheet shows blogspot receiving a small rise (12%) in traffic after their change, so that doesn't fit. But, clearly, something happened that was not just a blip.
A couple of months prior to that I took a part time job which curtailed my blogging a bit, but it did not affect My Quality Day, as I've maintained that as the highest priority with a post every day.
The reason I went exploring this question is that my Alexa rank has slipped on MQD as well, which seems odd, because traffic has actually risen a bit over the past month or so according to my Adgitize stats. I also learned that traffic has not increased according to Google stats, so there is some kind of discrepancy in those algorithms.
I offer no theories or conclusions. It's a bit of a mystery!