Never seen them in Beijing, nor Tianjin. I can't tell if they are only local to Guangdong.
Century eggs are eggs perserved in mixture of clay, ash, salt, lime, and rice hulls for several weeks to several months. Through the process, the yolk becomes a dark green to grey colour, with a creamy consistency and an odor of sulphur and ammonia, while the white becomes a dark brown, transparent jelly. (Wiki)
I was told not to eat many of these. I didn't like the smell, though.
I'm leaving Guangzhou in two days. I had amazing time here, enjoying the warm (and hot)weather, enjoying the green enviroment. As for the food, I guess I'm used to Tianjin's, the Cantonese cuisine is a bit sweeter than the one up north. I mean, it's all habit.
Have a lovely start to new week and month!