In The Future Humans People Will Be One Race: Averted.  It is perhaps possible (as the original trope points out) for this to happen naturally over a long enough time scale, but even the possible candidates for sticking around long enough for that timescale have generally taken to inventing about a thousand variations on their basic colors, shapes, and aesthetics long before arriving there.  Essentially, black and white are never going to naturally merge into beige so long as people are out there adding turquoise, ebony, gold-tinted, argent-blue, heliotrope and fur-covered to the gene pool.  And even if they did, someone would undoubtedly resurrect at least the look of the old phenotype for artistic purposes, if nothing else.

(Eldraeic racial divergence, while small to begin with and not manifesting phenotypically in the same ways ours does – due to a very small original gene pool, bioengineering, two bottlenecks, and more bioengineering – increased dramatically once they started inventing new ones all on their own.)

[The comment thread on the original posting of this trope read:

Comment: “Even if you mingled the current world gene pool thoroughly and didn’t tinker with the gene pool, there would STILL be substantial colour variations – there are some great twin photos out there showing what happens when one twin gets all the light genes and the other gets all the dark ones.”

Reply: “That’s true in the short, medium and long term, as the trope page points out – and, indeed, I’ve seen plenty of those Brazilian family pictures – but over the very long term?  One imagines once the alleles get scrambled enough the light/dark extremes are going to be pushed right out to the very ends of the probability curve, even without fixation.”]