They say shit OP.
In USA, 10% of black doesn't mean anything. Some huge part of the country are nigger-free. But in the south-east there are sometimes 30-40%.
It's the same in Europe. Most of them are in the west and for the bigger in France and UK. Always concentrated in big city. For example in France 70-80% of them are in the Paris metropolitan area (cf french scholar M.Tribalat). The higher the density, the more you've got. Like cockroach.
Tourists always see the big city, so they are in the cyclone's eye. Many other things can give you this impression of full blacked area. Mainstream media who always want to promote diversity when they choose a random guy to itw, white flight in school who let some classroom full of black etc…
In fact, even the more hardcore diversity propagandist, who want absolutely to say that France is already a multiethnic society, says there are less than 5%. A french afro lobby (CRAN), who need to exaggerate, pretend there are 3-8% with the caribbean creole. And France is one of the worst africanized country of europe.
It's already huge after all. And things change really quickly.