Henna loosened my curl A LOT.
I would think if you had very dark brown or black hair, this would make for a great solution without really changing your hair colour. The henna gives your hair a ton of shine and is not damaging, though some people find it drying. I think it depends a lot on how porous and/or damaged your hair is. I found my damaged ends seemed a bit dry or even slightly brittle. I think the henna just adds so much to my hair.. I went from a 3b-c to 2c and from fine to medium texture. My hair is strong and healthy and is finally adding inches.
If you have lighter hair and are not interested in having red hair, then you can look at other herbal treatments. I've heard Amla can help relax the curl but am unsure of this.