Braids can be good for black hair if you follow the necessary precautions to ensure they're done when your hair is strongest and healthiest.
I tackle the optimal conditions for braiding hair in: Hair Growth Tip 16 - Braids That Increase Natural Hair Growth
If you don't take these precautions you will incur lots of unnecessary hair breakage and thinning edges from strain and general dryness.
In addition, I should add that by mixing relaxed hair with braiding I damaged my own hair. I didn't know that it is best to choose one or the other. If you want to wear braids regularly it's best to stop relaxing hair altogether.
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