Q. What is the reason for using matte black zinc plating? L.R.
A. The typical reason for using a chemical conversion coating on zinc plate is for added corrosion protection. Using a black such coating does not, in most cases, give you any more corrosion protection than what is commonly called a clear conversion coating does. Based on salt spray testing, both the clear and the black will give approximately the same number of hours to white corrosion. Classically, a black chemical conversion coating (chromate conversion coating) contains a small concentration of silver ions.
With the advent of newer conversion coatings and the removal of hexavalent chrome from these formulations, the technology has changed, so you should talk to your chemical vendor about these types of coatings.