Back to Natural - Light Blonde to Dark Blonde

I would go into a Beauty supply store (not drug store since the boxes are not 100% accurate) I'd ask to see if they have a color book with actual dyed hair and ask the workers there for one that is "true to color". Then, take the actual hair and put it to clean roots. Pick one that is the closest Tone and Shade. If you need to you can mix two shades. You'd probably want to choose a lower developer (20? Ask the people in the store with your particular brand) so it deposits more and doesn't lift your natural/virgin hair very much.

I would also just do your whole head. I'm sure some professionals would disagree with me on that, but I've always had a lot of luck with dying-darker over colored and virgin hair. Also, it's all in the same "blonde" color group, so the line should blend pretty well.

If it grows out noticeably darker/lighter then try adding some mild low-lights or high-lights in areas so that it blends. (Think 1/2 a shade different from your hair color?) I know that is what a lot of professionals will do for people growing out hair.(Since you're doing it at home it is very unlikely that you'll hit it exactly on. Even hair stylist usually are off a bit.) Home highlights are a touch more difficult than all over color, I'd grab a friend or family to help you out.

... Luckily, I find going back to a darker natural color to be much easier than lightening. The only bad experiences I've had with hair color have been when I've dyed my hair darker than natural. (either from fading or trying to go back to natural)