This is why I think it is Germany. Perhaps this is representative of the whole country but particular to Berlin.
I come from North America(Montreal and New York). Though I see lots of bm/wf couples it is rare to see white women with black children. None of the WF's that I know have Black kids.
In Berlin I see at least a dozen white women with Black babies each day. I personally know 5 white women with Black children. I have never encountred this in North America.
Moreover, the last German woman I was with had the most interesting family.
Her married had a Black daughter from an African man. So this Blond and Blue Eyed woman has a Black sister. Moreover, this same Blonde and Blue eyed woman has a 3 year old Black daughter herself. (I'm not the father.)
So both the mother and her daughter both have Black daughters.
I do not know North America that well. I can only speak of NYC, Montreal, Boston and Toronto. However, Berlin has more white women with Black children than anyplace I have been to. I also know many Western Europena cities and so far Berlin takes first prize. That is why I nominate Berlin.