Black means you have never clicked on that thread before. Gray means you have. It’s just your browser telling you you’ve visited this link before vs. not.
White number in blue circle I’m not sure, but I bet @ace will pop in here shortly to set us both straight. I am aware of a little blue circle next to a thread indicating new posts since you last visited the thread.
One link I find very useful (and I’m not sure it’s prominently advertised on the site) is
https://talk.tidbits.com/?state=watching
which shows you all threads you have selected as ‘watching’, i.e. you posted there or you’re interested in following (per the selector at the bottom of any thread).
At the top of that list is also the very useful link
https://talk.tidbits.com/latest
which lists all active threads, also with a divider line since your last visit. A quick and simple way to check up what’s new since you were last here. Enjoy! 
Unrelated: @ace, how do I get an indent in a post? Leading spaces get omitted and hitting tab moves my focus to Safari’s URL field.