Actually, some ethnic groups can easily be distinguished by dress and customs
Sure, but I was talking about distinguishing them when they are naked.
Racial characteristics among Homo sapiens sapiens developed during thousands of years, between roughly 140,000 and 15,000 years ago, as the result of small human populations splitting from each other and living in total isolation from the rest of humanity for long periods of time (many thousands years), often in very distinct climatic and environmental conditions, which facilitated natural selection of particular traits of physical appearance among those individual human groups.
Distinction for three races - White, Yellow and Black - is oversimplistic and obsolete because Black race is in fact much more diverse than the remaining two ones.
What is called Black race should be further divided into several more races and sub-races. Bushmen and Pygmies for example are such races or sub-races. Ancestors of Bushmen separated from ancestors of the rest of present day's humanity 140,000 years, migrated southward, and for the next 100,000 years they lived in total isolation from all other human groups. Evidence shows that slow and steady gene flow between Bushmen and other human groups started again only 40,000 years - after 100,000 years of isolation, during which typically Bushmen phenotypes as we know them today developed. Similar situation was with Pygmies, ancestors of whom - as modern research proved - separated from the rest of Non-Bushmen humans some 70,000 ybp and remained in isolation also until approximately 40,000 years before present. Only then a steady gene flow between Pygmies and other Non-Bushmen Africans started once again. At the same time (ca. 35 kya / kybp) also some genes from an archaic species of Homo (their last common ancestor with modern humans lived ca. 700 kya / kybp) introgressed into Pygmy genome, but it was a relatively small admixture (2%):
http://www.pnas.org/content/early/2011/08/29/1109300108.abstract
http://www.abroadintheyard.com/ancient-africans-interbred-extinct-species-of-human/
http://www.eurekalert.org/pub_releases/2011-09/uoa-ahw090211.php
When it comes to the extent of diversity within "Blacks" compared to the extent of diversity between "Blacks" and "Non-Blacks":
Enough to say that if all of female mtDNA genetic diversity in times shortly before our expansion from Africa was 100%, then only 5% out of that entire genetic diversity actually ever paricipated in expansion into other continents, while 95% of it never left Africa (not in prehistoric times at least - only later, in historical times, in particular during intense sea-going slave trade). Moreover, out of those 5% only one part - that group which chose the northern migration route - became Caucasoids and Mongoloids, while the other part - that one which chose the southern migration route - became Dravidians, Negritos, Melanesians and Australoids.
Just like in Africa, outside of Africa there was also some limited gene flow between modern humans and archaic species of Homo. For example people of Caucasoid and Mongoloid races carry some Neanderthal genes resulting from occasional interbreeding. In total about 2% of DNA of these peoples come from Neanderthals, but it represents only 20% of overall Neanderthal genome, because only this 20% was apparently favoured in selection processes (it means that even if we gathered all surviving Neanderthal genes from all existing humans, we would not be able to "reconstruct" a full Neanderthal from them - only a 1/5 Neanderthal).
Blackness is rather more complicated than just looks. Plenty of Blacks don't look Black but are Black according to the American understanding of Blackness.
I agree with this American-centric argument. But you should know that Afro-Americans are not "pure Sub-Saharan Africans". Afro-Americans have on average a significant degree of European admixture, as modern research shows. Paternal European admixture is disproportionately higher than maternal, resulting from intercourses between white slave-owning males and their black female slaves during 250 years of slavery in North America. So despite this strong "African Black pride" among many Afro-Americans, most of them are to some extent Caucasoid / White. Also recently there has been an increasing number of interracial marriages.
All in all, Afro-Americans are 10% White and 90% Black (on average), in terms of overall genome. Among some individuals percent of White genes can be much higher of course. While in terms of Y Chromosome (inherited from father by son), 30% of Afro-American males carry European versions of Y-DNA. By contrast in terms of mtDNA (inherited from mother by children), only 5%. See the link below for more details concerning Non-African genetic admixtures in African-Americans:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3051415/
And this graph:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3051415/figure/fig02/