OK, I had a wee look at Point Color in Camera RAW. I was able to constrain the effects to the dress without desaturating her skin tones, but I couldn't fully desaturate the dress, or get it as near to black as Hue/Sat could do. I think it's more subtle and designed for fine tuning photographs, and not the sledge hammer to crack a nut that Hue/Sat was.
Check out Colin's video on Point Color if you are interested. Heaven knows ACR 16 is so different I struggled to find my way around it at first, but I really don't d much photography nowadays.
The way I have always done it is with a Hue/Saturation layer.
Use the little hand icon top left of the Properties panel to select the dress.
That limit the adjustments to just that colour range, as indicated in the colour barsat the bottom of the Properties panel.
This is problematic in this case, because the young lady's face turns out to be a similar, albeit less saturated colour, so we have the use the Hue/Sat's layer mask.
You are looking for grey or black, so move the saturation layer all the way to the left to get grey, and the Lightness slider to the left to get black.
I watched a video on Colin Smiths channel about the new Point color earlier today. I wonder if that would do it?