But for what I did find, or already knew...
The slabs were sold to me as "Black Amazon with Gold" granite. It is not a granite at all (though some of the pebbles in it are): it's a metaconglomerate. A conglomerate, fundamentally, is a bunch of large pebbles to cobbles that are glued together somehow, usually with something finer grained like a clay. If you take that conglomerate and subject it to heat and/or pressure (say by burying it under a few thousand feet of other sediments, or by having a tectonic plate collision squish the area up some), the rock gets metamorphosed into a metaconglomerate. You can see the big pebbles in the icon photo, and might be able to see that some of them are a little stretched out by the metamorphic process.
So, today's search uncovered that this is more commonly called "Marinace", and probably hails from the state of Bahia in eastern Brazil. There's a very popular variety called Verde Marinace, but that one is decidedly greenish and mine is not. Found one site that showed a picture labeled Nero Marinace that looks like mine, or other sites that just call it Marinace. What I'm not sure of is whether "Marinace" is a generic term for conglomerate (and/or breccia, which is the same sort of thing with angular pieces instead of rounded ones), or if there are specific suppliers of each and thus maybe a specific geologic formation they come from. Or maybe Marinace is specific to Brazil, but still encompasses a range of rocks.
One last tidbit is that the Verde Marinace is Precambrian age. Which may not tell me anything about my countertop stone, of course.