Mine's been blue since the release of classic format. It's independent of account, platform, or installation so I think it's just a "feature" for everyone. I hope they remove it. Maybe we can pay 5 gold to remove it? That's also been bugging me for a while
I was quite puzzled that they didn't fix that with the last patch (assuming it's a bug, not a feature). Hopefully will get fixed with the next one. It looks so annoying. But if that were the only complaint right now, we all would be quite lucky, right?
I thought it was supposed to highlight that there's a new game mode, just like the Tavern or Adventure tabs are blue when there's new stuff, but I've played Classic games since and nothing changed, so I guess it really is a visual bug and a very annoying one.