Does Zinc become black & dull after reacting with dil. HCl?
Look at it under a microscope.  If the surface is microporous, light goes into the reflecting cavities, bounces around, and is absorbed before it can escape.  Wilkinson Sword sold bolted stacks of double-edged razor blades as low power laser beam dumps,  Each sharp edge was mirror bright with a slight taper toward the edge.  Light bounced in and could not escape.  The sharp sides of the stack were astoundingly black.  375 blades x .004" = 1.5" square target.

Or, you have impure zinc and crud is accumulating at the dissolving surface.  Alloy 8111 aluminum foil contains about 2% or so of strengthening dispersed intermetallics.  Dissolving aluminum foil in acid or base leaves black residues.