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Magic in Homestuck is a broad topic. There really isn't anything that separates John's stage magic from Rose's forbidden occult lore from Roxy's old men with beards and robes. It's kind of like how Light represents luck, and also other things simultaneously but you really shouldn't try splitting them apart. Much like real life, trying to get a strict definition of magic is exceedingly difficult.

Magic also coincides with belief. Not just the belief in magic, but where and how characters put their faith. Vriska believes in herself, so she has all the luck. John isn't quite as self-confident, but has complete faith in his friends which makes him the leader that other people can believe in. Eridan clearly does believe in magic but tries to deny it; leaving him with warped perspectives and priorities.

Most importantly, magic enhances but never provides. There is a contemporary view on magic as being "special powers that let you do whatever for no reason" popularized by somebody I hope dies in the near future. Few characters in Homestuck expect this out of magic; and they all fail because of it. The most successful magic users are those who supplement their own practical efforts with magic.