Most Helpful Girl
I've dyed my hair several times, once including accidentally going black so I'm pretty familiar with how to best add colour to really darker shades similar to what you're describing.
Dying your hair blonde at once is more damaging to your hair than brown, as the lighter you dye it the more hydrogen peroxide there is which is the chemical that ultimately does the damage. If you do it just once or twice a year though, it shouldn't really make a difference in how healthy your hair looks.
Also, black is the hardest colour to dye over. Keeping that in mind, if you want your hair to go chocolate brown I would use a brown hair dye a few shades lighter than what you want it to end up as.