You're anti-pink, and want a superhero costume! I like your style!
One of the first problems you may encounter is the penchant for comic book artists for FEMALE heroines/villains to dress almost exclusively in shades of black, in costumes that seem to be painted on (not necessarily a complaint given some of the actresses that then play those heroines on the big screen, but I digress). There is a list of heroines available which should make my point quite well (regarding the black/skintight thing anyway... they're often quite obscure with names like "Lady Death", or in one case "Stripperella" (which is as bad as it sounds!)
So moving on to recommendations, if we discount stuff that's either too niche, based around vengeful pole-dancer costumes, or both, there are plausibly 5 options that are both presentable and well known. (Unless you want to spend your entire party explaining why your kid is wearing blue body paint and a loincloth (some comic books are quite obscure!).
- Super Girl / Spider-Girl / BatGirl. Female equivalents of their male counterparts with almost identical costumes. (Superman/Spiderman/Batman). Making this yourself could be a pain, but ToysRUs sell romper suits in all these patterns that are fairly unisex.
- The Yellow power ranger (as the other female one is pink). This would involve dressing in a bright yellow romper suit with hood/sunglasses. She'd get daggers to play with as well!)
- Wonderwoman. Perhaps the easiest and most statemented costume to DIY. Blue shorts with glitter on, a red top, wide bracelets and a tiara!
- Something non-superhero but still fantastic. Have you considered a pirate theme? (Jaunty hat/waistcoat/frills), Prisoner (go Guantanamo and bring a hooded baby in orange jumpsuit!), soldier (girls can be soldiers too... it's like being a superhero, but the clothes are baggier, the bullets hurt a little bit, and it's usually the men you work with coming out with the innuendos rather than the evil mastermind!)
Of course, you could go gender neutral with an Incredibles outfit (red, black boots, black circle with "i" on chest)
Hope this helps.