As you dye darker, only go two color levels at a time. (from the darkest black to the lightest blond there are 10 levels, some colors will say "level 6 dark blonde"-look to them for guides).
As you darken, you need to fill in the underlying tones of yellow, red and blue to get a healthy brown color, by darkening in steps, those colors will be added properly. (if you've ever seen someone with brassy/orange highlights-you know there is red and orange under there).
How is your hair? Is it healthy? Have the highlights damaged it? You may consider doing a conditioning treatment before coloring (leave a conditioning creme in your hair for 15 min with a shower cap on-if you can, heat with blow dryer, then wash out with cold water).
I've tried lightening at home, no luck, so I think you're making the right decision.
Good luck.