It could be done with enamel or even some kinds of paint. It could also be done by using silver and gold (silver for the parts you want to blacken) and using liver of sulfur on the silver portions. The designers who are telling you it can't be done OP boggle my mind a little though b/c blackened gold is not unheard of and there are lots of ways to get that effect.
If Cathy Waterman's bench can do it, any decent bench can. I say this as someone who has owned a few of her pieces, and the craftsmanship is not the best.
CvB is able to do it:
David Klass can also add black rhodium plating to pieces.