Someone who rides inline skates.
Comes from the name of the well known company Rollerblade.
Usually meaning someone into "agressive skating", which means people doing tricks on skates (slides, jumps, in places like streets, ramps or skateparks).
It's the equivalent of skater
(skateboarder), but on roller-skates.
Like for skaters, there should not be any link between this word and a specific music, clothing style, skill level, or whatever : It just describes someone who likes to have fun on inline skates.
"I met a new blader in town, he told me he would ride the park this afternoon, wanna come?"