I contacted a local grain/flour company, and they have a few different cereals in stock. One of them was described as 'Aveia Preta' or 'black oats'. I looked it up, and seems its a smaller brother/cousin of typical oats. Nothing mentioned here on the forum according to my search, and not that much detail on wikipedia either.
https://en.wikipedia.org/wiki/Avena_strigosa
It's used mostly as a cover crop here in Iberian peninsula, just fixing nitrogen and keeping the land good between other harvests, but the oats are a kind of byproduct of that.
I've ordered some and will report back with my findings!
Black Oats (Avena Strigosa)
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