Follow up.
The Scottish gruit thing is all gone now. Interesting brew, which turned into an odd herbal stout-ish brew as time passed and the stash dwindled. Too much carb, in spite of low dosage of priming sugar. Not sure what happened there, but the beer was okay and interesting in spite of this.
The second runnings turned into very serviceable Flemish Oud Bruin sort of beer. I added raspberries and frozen pitted cherries for about 8 months.
Very dark, almost black, with reddish highlights and a pinkish head.
Very sour, with acetic acid noticeably present. Think single-foeder FOB à la Rodenbach Vintage in terms of sourness.
Very enjoyable. Uncarbed, the dark roast interfered a bit with the sourness and the fruit, but with mild carb, this turned into a lovely sour.
Since it was no longer black, I re-named to Old Sock's Legacy, in honor of the master of sours himself.