Oh I didn't know H&M has a hair dye?? It's not available in Poland. Now I'm really curious :) I'm also doing my roots only not to demage the whole hair.
When I went blond I had naturally dark hair (dark brown) and some lighter higlights, but not dyed black. So it was really easy to became a blond girl ;) Now I decided to stick to dark / black, but I have some blond wigs for parties and "special occasions" ;))
When it comes to short blond hair I like Kate Lanphear's hairstyle.
Ohh..I also have a great pic of blond hairstyle that you may like, just need to look for it at my harddrive. :)