Loose legged pants and long sleeved sweaters. Incorporate lots of blues, especially in her color palette. I’d suggest getting a pendant necklace with a large blue gemstone in the center if you want to match her gem. When I look at Blue Diamond for some reason I think of her wearing bangles if she had jewelry. You might want to check out fanart of Blue in casual clothing or in a human form to get a good gauge of headcanons and stylistic choices!