I know this should be obvious, but I'm really struggling with it. I've tried all the basics like just painting her hair brown and changing the blending layer, or creating a fill layer, or using colorize in Hue/Sat, with no luck. All of the YouTube tutorials and such are for changing brunette's to blondes, not visa versa. Her hair is just so whited out that I'm struggling to pull any color out of it, and when I colorize it with browns it just looks awfully fake.
Any ideas on a clean way to turn very white blonde hair into dark chocolately brown?
I'd look into a variety of things such as: selecting the hair and creating a new layer, filling it with the brunette color you're after, changing blending modes, masking areas if needed, dodging, burning, and possibly adding additional duplicate layers until you achieve the desired look. Adjust levels, curves, whatever until you get what you're after. You'll probably find it'll be a combination of tools/filters/etc to get what you're after and to make it look believable. One thing you can consider is to bring in a target image of the brunette hair look/color you're after, so, as you're editing, you have a reference to go by.