Interesting bit of information from threepinner in the Flickr DIY Color group, here. (See? Sometimes it's worth keeping up with Flickr - you never know what needles of genius will be found in the haystack of nonsense).
I'll give it a try with some pound-a-roll film initially. It does seem to enhance black clothes, for example here. I'm envisaging it as emphasising the blackness of the background of a colour exposure with flash, outdoors at night, as it seems to be doing, here. Essentially, it's the addition of an alkaline bath to the process, between dev and blix. Underexpose a couple of stops, too. No filter.
Be interesting to see what it looked like with xpro.
This led me to google "Stoeckler Two Bath", (never heard of before, in our house) and get the formula here. Blimey. How simple and cool is that?