Oh please, swarthy (strange word BTW) isn't black.
And those darker people in Germany will have Romans as ancestors (or maybe Roman slaves, and those could have been from all over the empire, incl. North Africa and the Middle East). Don't forget that the Black Forest area was part of the Roman empire for ~200 years.
Not a big deal to me, but the way I read it is that the Black Forest
people were, as late as the middle ages, distinctly dark. Yes, if any of this is
correct, certainly Roman in part. But that is not the issue, genetic
evidence by superficial methods is.
For example, blonde hair in Ireland. As heard, it is still used by the
local Irish population in the rather negative slurring blondes, the form
of persons having "slow ancestors" meaning they did not run away
from the Vikings fast enough. (Red hair is a Celtic/British Isle strong
point, though, the highest ratio in the world is with some western isles
of Scotland, about 14 -16 % I recall.)
Having distinctly swarthy color is harder to ignore, usually harder to
breed out, and a quick death sentence at many hard times where any
negative identifying issues are dangerous. So head for the hills where
few people venture. The Black Forest is very lush, even in today's
acid rain supports easy to hid, easy to grow things (comparable to the
alps, which was the bastion of another group running away from the
Germans, the Celtic Helvitia (sp).
And the Black Forest was known for storytelling and deep German
customs. When people are against you, your own culture, and ones
you must at least occasionally trade/interact with, adopt super
culture and people will accept you more, tell you if anyone is looking
for you, etc. Easy to lay an ambush in the Pacific NorthWest, as those
forests are incredibly tough to make good time or see a few feet infront
of you. Both the Black Forest and the Pacific North West get lots of rain,
thick undergrowth.
Finally, on the subject of racial mixing, it does happen that one parent
dominates a great deal, capable of passing for that parent's race. But
future children could activate that gene, especially when two offspring
themselves have children. And one child deep in the woods can be
hidden, not so in the lowlands. We see the same thing in the
Western Carribean, even though their lives were not in danger in living memory,
with the mostly white Creoles. It was a time life type book which mentioned that
very same issue (one sentence plus picture, of a twenty volume series I have seen many
times in libraries all over the US/the Globe since coming out circa 1977.)