The problem with bleached hair is that when you are going to a darker color you are missing certain pigments. When you go to a salon they will do a filler, the filler is going to fill in the pigments that have been bleached out of your hair (the reds/oranges). When you apply a darker color to your hair without filling it can look hollow and 'off.' I did a color correction project in school where we did this exact same thing. The portion that I used a filler on came out near perfect, but when you look at the section of bleached hair that I did not fill before the color, you can tell it's just not 'right.' IT's not horrible, but my Cosmetologist eye can definitely see the difference. If you don't have the money to do it, just grab some box color and hope for the best is all I can say, however there is a chance that the root area is going to be noticeably different in color, especially when it grows out further.