Greg, you may be right about the separate, stlylistic origins of Black Beer. There were numerous black beers in continetal Europe that predated Porter, including one that was based in Flanders that was revived in Colorado I believe it was, the 15-- something beer (with a year denoting the era of currency of the style, I can't recall the full name of the brand).
Also, in more modern Flanders, there were black (and red and white) beers - we all know about the white - so-called in the 1800's and earlier. My source for the latter are notes on historical beer styles in French Flanders by Pierre-Andre Dubois in a magazine published by the local beer lobby, Les Amis de Biere.
However I believe that traditions may have merged: the original German black beer one with the later porter one with the later lagering trend. Porters were famously influential and admired in Eastern Europe...
The Kostritzer has a leathery note and also a roasty note that remind me of many porters, too.
Now a potential irony: could (a presumed) Germano-Flemish black beer have been itself the source for Porter? I have always felt this was possible. The Flemings came to Britain in 1500's or thereabouts and famously brought hops, but they could have brought more, a taste for specific styles, and their terminology. In The History of the Pint, it is suggested that AK, a beer style of which only one brand is now extant, probably stands for "a__ keut", a relatively low-hopped, low AVB beer current in Flanders for generations. (The blank space is because I can't remember the first Flemish word in that name but it is in the book). I have seen in separate materials this reference to keut beer in Flanders, its one-time existence there is unquestioned. No apparent connection between keut beer and porter or black beer, but I mention this because it may show some definite influence of incoming Flemings on English beer styles.
I have always felt, although Michael Jackson disagreed and told me so once in person, that the Belgian Poorter sold in those stone crocks - I can't recall name of brewer at the moment - was a porter-like beer and perhaps its ancestor stimulated the development of Porter in the early 1700's. This might be something as simple as, the word porter might have been a dialect word in England for a roasty brown/black beer, brought over by Flemings, and when three threads/entire emerged as a style, it became dubbed by this old dialect or cant term. The whole thing about porters (carters, I mean) loving the style and the name coming from them always struck me as fanciful.
Anyway, see ya all tomorrow.
Gary