Sinceramente, creo que por mucho que nos haya subido el Ibérico a todos, me parece una actualización muy extraña.
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Don't you think it could be because there has been more movement of people both to and from America and your African colonies, than in the case of Spain with its colonies?
That is to say, not black slaves mixed with Portugueses in Portugal, but Portuguese who had lived in the colonies for some time, even several generations, and returned already mixed from there.
Or they went to the colonies for a time, married people who had been in the colonies for several generations and already had some foreign contribution, and returned married.
Just to offer more options, without invalidating what you said about former slaves etc.
Los canarios suelen ser rubios.
https://i.imgur.com/ux7ZQoR.jpg
The first record of black slaves in Portugal dates back to 1444, in Lagos, Algarve. They kept going here for forced labour, for the next centuries, not in the percentages or numbers of the Americas but in some cases enough to make a little genetic inpact (1-3%).
I have this teory that minor SSA is mostly present in Portuguese living in the coast or with a familiar background from the coast, I for example have only family from inland Portugal tracing all back to a minor village in central Portugal quite close to the Spanish border, and I score 0% SSA in most calculators, or very minor (0,3-0,6%).