Think back to the early 2000s, the era of boy bands, MySpace, mirror selfies, and hair that was colored in your basement sink with a bottle of bleach and peroxide, à la Christina Aguilera and Beyoncé.

There is a long-held belief that natural hair cannot be bleached. Because curly and coily textures are prone to dryness, many in the hair-care industry have held that these textures are too fragile to ...