Why topic headings in blue or black?

I was just wondering why some of the topic headings are in blue and some are in black. What is the difference? I have viewed all the topics and see no difference to account for it.

If you are referring to something similar to what I show below… mine are black(completely new topic or further posts added since last viewed) or gray(all of the current posts in topic have been viewed). I will note that if after you have opened the topic for viewing and read down to the bottom, upon exiting you can find that the topic name is not grayed out, because while you were viewing that topic’s posts,… another reply was added to that topic and not presented to you at the time of your viewing.