Is it possible to imagine a scenario where the West stays Lilly white until today? And White Australia and the immigration act of 1924 essentially goes on and on forever? Like if the Nazis implode after 1933 after the Reichswehr declares a coup, so no WW2 (at best a minor German Polish border war that results in Poland losing its Baltic access), and white racism and sense of moral superiority, and low grade 1930s antisemitism and eugenics basically go on and on. Basically in this scenario, the Great European powers may still lose their colonies, but instead of the British or French deciding on mass immigration, they decide to wall themselves of from their former colonies after losing India or Vietnam.
The question is: why would they? More specifically, why would they not start
gastarbeiter schemes, recruiting non-European labour force (*) to counter labour shortages in Northern/Western European industry and mining. After all, it's here that the original basis of non-European emigration to Europe was laid - it was Europe, or at least Europe's industrial groups and governments, that initiated this, a fact certain xenophobes now like to forget.
Barring gastarbeiter programs, the labour shortage problem would have to be resolved through other means.
(* Yes, I know that in many conventional racial pseudo-classifications, North Africans and Turkish people would be considered as being white or at least "caucasoid", I'm however assuming that the OP doesn't and that his question is more generally aimed at non-European immigration to Europe.)
Say France and Belgium still loses Algeria and Belgium, but after accepting the Peid Noirs and Belgian exiles they decide to raise the drawbridge in this scenario.
A sidenote: Congolese migration to Belgium, although certainly not nonexistent, only really began to get at full speed during the late 80s and the 90s; in 1981 there were barely 8 575 Zairians in Belgium, mainly students, businessmen and political dissidents. And even now, Congolese or people of Congolese descent are only a small part of Belgium's migrant population, albeit a visible part (there's the Matonge neighbourhood in Brussels + younger generations have recently gotten more vocal, which I applaud). The link between Belgium's current multicultural reality and its colonial history is real, but less direct than one might think.
To illustrate this, in 2006 15% of Belgium's population (1 625 362 persons out of 10 511 382) was foreign-born, the two largest groups being Dutch-born and French-born people (although those might be themselfs of non-European descent).
40 301 persons out of those 1 625 362 were born as citizens of the DRC (or Zaire) - that's slightly less than 2,5% of Belgium's immigrant population and 0,38% of the general population.
Of those 40 301 DRC citizens living in Belgium, only 9 048 people were born in Belgium.
The actual number of people of Congolese descent is probably somewhat higher, when DRC citizenship hasn't been transmitted to the next generation. Note, however, that these numbers do include those who have become naturalised Belgian citizens (about 2/3 of them). In any case, it does form an indication of the relative (un)importance of Congolese immigration to Belgium. Given migration trends since 2006, the relative importance of the Congolese immigrant community in Belgium has probably diminished.
(More details can be found in this study on Congolese migration to Belgium - unfortunately only in Dutch and French:
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http://www.myria.be/nl/publicaties/congolese-migratie
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http://www.myria.be/fr/publications/migrations-congolaises)