A blader is someone who skates on blades — most often inline skates, the ones with wheels in a single line that many people still call Rollerblades (that began as a brand name). You can also hear it for people on ice, but the street and park meaning is the one that stuck in hobby talk: the person rolling along a path, jumping a kerb, sliding on a rail. It is informal sports language, not a job title for a form. The fuller word is rollerblader; blader is the short, in-group version. Though actually, “skater” on its own often means a skateboard unless you can see the wheels, so blader is useful when you mean the inline-skate crowd.