The main character, Kenshin Himura, is a former assassin who wanders Japan during the Meiji Restoration period. He's known for using a reverse-blade katana and often wears a red cross scarf. While he doesn't use rollerblades specifically, he does use a technique called "Hitokiri Battousai" that utilizes fast, agile movements similar to those of a rollerblader.
This is a popular anime with several different versions, including a movie, so you might be thinking of a specific adaptation. Could you give me some more details about the boy and the alternative world he enters? This would help me narrow down the possibilities!