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Diet
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In captivity, marine fish almost always eat different food than they would in the wild. Sometimes this means that fish do not get all of the nutrients they need in captivity, which can cause a fish's black pigment to fade. You can try to correct this by feeding foods rich in caratenoid, vitamin C and highly unsaturated fatty acids or HUFAs. These include foods like grocery store seafood and nori.
Stress
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In addition to poor diet, stress can cause a fish's color to fade. A number of things can stress out saltwater fish. For example, incorrect water parameters -- like the wrong pH, hardness or temperature -- can make fish feel constantly stressed, leading to their black pigments fading to gray. Additionally, sharing an aquarium with aggressive tank mates can stress the predatory fish enough for them to fade.
Disease
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Related to stress, disease can make a fish's black pigment fade. Fighting disease typically taxes a fish, causing it to fade. More directly, some diseases actually attack a fish's skin directly. In this case, black pigment -- and the skin itself -- may be damaged or covered by parasite colonies.
Other Changes
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Fish may change color for some reasons that aren't problematic. Many species of fish can change their color to some degree and will do so to camouflage themselves to hide from predators. Additionally, some species change colors when they breed, whereby more colorful pigments take the place of black pigments.
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The Loss of Black Pigment in Saltwater Fish
Fading black pigment, or a general loss of color, can signal a number of causes in saltwater fish. Many fish fade when stressed or diseased. Other fish change colors to camouflage themselves or signal readiness to mate. You need to understand why your fish lost their pigment before you can take action or relax.