Hi EllenNicolle, I have naturally jet black, relaxed hair too. I decided to get highlights professionally done for my wedding. Thanks to Tracy, the hair color guru, I did not have to suffer breakage.
The main points I came away with were to be diligent about upkeep and vigilant about my hair's needs day-to-day.
Warm oil preshampoo treatments helped as did light-colored rinses from Sebastian's Cellophanes/Colorshines. I had to oil my highlights specifically and I found shea butter did the trick. I also used Feria's Color Moisturizing Treatment conditioner (by L'Oreal) at Tracy's suggestion, which made my highlights sparkle.
Finally, I found that *my* highlighted, relaxed hair required more protein than when it was only relaxed. I upped the protein conditioners to once a week and haven't looked back since. I like Aubrey Organics GPB and Elucence's Extended Moisture Repair Treatment because I have found them to strengthen my hair yet remain moisturizing. I continued to use moisturizing conditioners and leave-ins.
Sorry this is so long! Pros: very pretty, new look; Cons: more serious upkeep, greater risk of breakage due to the double process of relaxer and color.
HTH