This is all default Discourse behavior. Haven’t done any investigation into changing the styling on this.
But looking at this example, the blue dot indicates that a post is totally new to you. You have never clicked into it. These are the posts that will appear in the New tab. If you hover over the blue dot, it’ll say “new topic” (not very discoverable, I know).
I believe that black text and no new indicator means that you have previously looked at it, but there are either new replies or you never read the entire topic. And grey text indicates that you’ve read everything in the topic and there have not been any updates.