If you haven't tried this already, then make a new Curves Adjustment layer, and lower the RGB curve as much as you need, then change the Red and/or Blue curves to suit your needs as well. Once you get the right effect, mask the layer so it only affects the hair. I think the main difference between blonde and brown is that brown is simply darker. Normally it should be enough to just lower the RGB curve...
If that doesn't work, use a Hue/Saturation layer and check "Colorize". Then you can change the Hue slider to get any color you want, and you can strengthen or weaken that color with Saturation. But don't use the Lightness slider!