I thought that if you applied too much current when nickle plating the finish would burn and darken. Nickle is inactive or inert till activated but you might experiment a little. Activate it and add a few microns of overcurrent nickle.

It almost sounds though that if the customer wants the finish black he might have to supply it that way. Black chrome is also a finish for example. I still have a partial roll of copper with it as the final finish left over from construction of my solar panels years ago. It is an electroplating process.

The issue is to do these things reliably is not really cheap in my opinion.
Maintaining long term conductivity has to be a consideration as well. I assume the customer desires this for esthtic reasons as I can not visualise any other neccesity.