That's certainly true for Phippsburg Land Trust trails.
But the Appalachian Trail is uniformly blazed white. Side trails are uniformly blue, at least if they are blazed according to the trail manuals provided by the Appalachian Trail Conservancy.
I also disagree with the claim that thru hikers need only be concerned with white blazes. I took most of the blue blazes that were purported to go to scenic and other interesting places. I figured that some maintaining club thought the side trail provided something special, and it would be impolite not to take a look if it was a good day and I had time.
Weary