Most leather is dyed, even brown leather.
But black dye, I’ve found, takes much more time and application to sink into the leather, and is more obvious when it fades. I prefer to only use non-abrasive, gentle soaps on my black tack. Don’t over-oil it (which can saturate the fibers and cause early fraying), and try to store it in climate-controlled places. What disrupts the dye is microtears and breakdown of the collagen fiber in the leather - caused by either being too dry, or being too saturated.
I have found that in the cheaper black tack, the dye can fade easily (especially around the noseband and cheek-pieces) - so I avoid castille soap (despite it being my favorite soap), as it can be caustic. I also never use ammonia or dawn/dish soap on black tack either.
Tack being black doesn’t have to necessarily indicate poor quality. And being dyed black doesn’t mean that it’ll be less durable than its brown-dyed counterpart. I have a black Nunn Finner breastplate that is higher quality leather & craftsmanship than my (brown) Vespucci. My Courbette bridle is black and is still as dark (if not darker) as the day I got it… and it’s probably ~20 years old at this point.
Quality black tack will stay black if you take good care of it.
For saddles, it can be a little more difficult as they see much more wear on them than bridles do. I prefer to avoid standard glycerin bar soaps with black saddles, personally. Most black tack will do very well with a softer type soap like Belharra saddle soap.