Peter, he just won't (can't?) let go of that obsession, because it seems to be intimately tied into his self-concept.
As for the rest of us, *we* don't have to believe it. There is plenty of evidence that Europe has been predominantly "white" for an absolute minimum of a thousand years and probably many times that. Take the Bayeux Tapestry, for instance, reasonably dated to the 1070s and celebrating the victory of William and his (Norse-derived) Normans over Harold II and his Saxons. You will not find a single Black face in it from one end to the other. (You *will* find a reference to a, probably sexual, scandal that was common knowledge at the time but has long since been forgotten.)
Or take the various Books of Hours, very popular with the nobility from c. 1250 to c. 1650 (give or take a bit either end). The illustrations show saints and angels, princes, prelates and peasants - and just about every single one of them, bar occasionally one of the Three Magi, is "white". (There developed a tradition in the later Middle Ages that Balthazar, the one of the Three Magi/Kings who presented the myrrh, was a Black African. And yes, by then they knew what Black Africans looked like, and rendered him with reasonable accuracy.)
We do not need to know Somerled's forefathers. We know his *descendants*, numbering in the tens of thousands. The vast majority of those who have been tested have come up R1a and/or closely related subclades of that haplogroup.
R1*a* is not the commonest haplotype in Western Europe - R1*b* is. And for it to be *that* common, it must have been there for a *very* long time, several thousand years at least.
The Romans named the Picts (we do not know what the "Picts" called *themselves*) because of their habit of painting/tattooing themselves blue with woad-based dyes. They were the "Painted People", to the Romans. Woad shows up very well on light skins, not so well on dark skins. (Australian Aborigines use white clay for skin designs, for essentially this reason.)
Just because someone acquired the byname "the Black" doesn't necessarily mean that they *were* Black in the modern understanding of the word. Thorfinn Sigurdarsson, greatest of the Norse Jarls of Orkney and a contemporary of Macbeth, is thought to have been called "the Black" because he was black-haired and a very doughty fighter. (He was also biologically at least three-fourths and probably seven-eighths Celt. And the Celts tended - as both Greeks and Romans knew and reported - to great stature, very fair skin and, usually, blond or red hair.) The "Black Douglas" was so called, not from his complexion, but from his bad temper and violent disposition - "By and rade the Black Douglas, And wow but he was rough!"