I have recently encountered this problem and have made a post elsewhere. I will do a quick reply here.
Typically, the absence of color should imply an "absence of activity." Items not read should be white, not blue. Furthermore, I would argue that the system should not be changing the color because only the student knows if they have really read something. So an Announcement (or discussion post) should have a white dot that only the student can change to blue, meaning that the student believes they have read the post.
I was equally confused with Canvas' behavior as Mary. At least now I understand this (faulty) logic.
This discussion post is outdated and has been archived. Please use the Community question forums and official documentation for the most current and accurate information.