Excellent question. It is actually one we dentists/orthodontists all asked ourselves. For years we have preached "protect enamel." We warn of wearing down teeth too much. Many bad things can occur if enamel isn't worn down properly; sensitivity, decay, crowns, root canals, etc.
There are a few truths to know that will put your mind at ease, when addressing a black triangle.
1) processed foods in our diet
2) cavemen
3) weak enamel
4) fluoride
Over the past century we have has a huge increase of process foods. The typical caveman diet (now in vogue again) was mainly of nuts berries and carcases. If you even have the opportunity to look at a caveman skull, notice their teeth. You will see not only are there more teeth than we have now, you'll see that they are very square and perfectly aligned.
The reason is our teeth we built to last with a caveman diet and be wore them down "interproximally". On average we actually have several mm of enamel surrounding the tooth, before any real harm is done. As orthodontists and dentists we can take advantage of this to achieve a much more cosmetic, often more hygienic and more stable result by preforming mild interproximal reduction (IPR). {Results are more cosmetic bc the black triangle are less noticeable or gone all together. Hygiene can improve bc food gets caught in the black triangles less and less surface area for plaque to accumulate. Stability is your long term orthodontic result is much more stable when their is a solid broad contact compared to a previous pin point contact.)
Some studies have actually stated that by doing IPR you are likely removing weakened (acid attacked) enamel and allowing for a more fortified enamel wall to protect the tooth. To insure that this is the case continue using a fluoride toothpaste. Anything with the ADA approve logo on it is acceptable, floss floss floss (they even have fluoride fortified floss) and a fluoride risen is never over kill.
Keep in mind black triangle will worsen over time because of osteoporosis so taking calcium is always a good thing too.
Word of advise: make sure you can visualize the tooth not looking too skinny prior to doing so, triangular teeth tend to be the best for IPR. Bonding can be another viable option for a black triangle if you'd like to add to the tooth and not take away from it.