I have long hair past my shoulder blades. From my blades up to my ears, my hair is blonde from previous bleachings. My natural medium brown roots have grown out quite a bit and my hair is not toned. It looks very dingy and stringy because of the mixed tones. I want to go darker but I want my hair to have even color distribution without further bleaching. What shade/color/method should I use to achieve an even coloring?