Is rollerblading a good cardio exercise?

Far more popular outside of the United States I can tell you that much. It has a massive following all over Europe, parts of Asia and then more in Australia. Rollerblading is virtually existent in most countries though in smaller quantities. Skateboarding is still, by far more popular due to it's massive advertising campaigns, visibility and representation in most media, it's more than 40 year history of existence and its attractive culture that most other sub cultures like it have several common similarities. I hope this helps answer your question.