Somehow, I guess because of their light coloring, blondes usually don't look so hot when they go brown. Black, sometimes, if they're very pale. Women with light coloring and brown hair, can usually go blonde without looking like a ho. Women with olive skin... 
I think it depends on the colorist. I know two women who are natural brunettes that went blonde for years,
...and no one had a single clue. One woman was blonde ever since I met her, from 1987 til 2000. In 2000, I saw her with dark hair and assumed she died it. Turns out it was her natural colour. She stopped bleaching when he got pregnant and decided to let it grow out naturally. The other one, I automatically knew she coloured it. I ran into her on a commercial shoot and was blown away. both those women, you would never be able to tell. their hair was healthy, had dimension and highlights. It wasn't that brassy, flat, all one colour look, like so many bottle blondes, theirs had dimension and depth, like natural blondes and they had no roots, ...EVER!
I don't care how badly you want to be a blonde, ...if you can't do it without turning your hair into a one dimensional brassy clump of straw, ...don't do it, ...and for gods sake... touch up your roots.
