It seems you misunderstood a bit, which is my fault, so I'll explain.
Facial blindness for racial features occurs when you're not accustomed to having to noticing such features. Most homogeneous regions can readily identify outsiders: they just look different. They're less likely to be able to identify where the outsider is from.
Guessing from facial features, with great but not perfect accuracy, is a skill. Like any skill it needs to be practiced. It is also never 100% accurate but I'm right most of the time.
Korean people have rounder faces and a less distinct chin than Japanese people. Chinese people are a bit in-between the two in terms of head/face shape. Koreans tend to have smaller noses and less pronounced cheek bones. Japanese have thicker eyebrows. The "double eyelid" is more common and prominent in China/Japan than Korea. China varies a bit more by region - but the people from Guangdong look different from those in Jiangxi or Gansu.
As a rule of thumb the north-south effect applies to Asian just like it applies to most of Europe: people in the northern regions are on average taller with lighter skin than people in the southern regions.
It also isn't just East Asia. Most people have difficulty telling African people apart because it isn't a skill most people need to practice. In East Africa, (eg. Uganda, Kenya, and Somalia) people tend to have larger foreheads (or more accurately: higher hairlines) than in other African countries. I actually learned a lot of this from watching a linguist, who often goes out to speak with people in public [0]. To approach and open a discussion he often has to guess their ethnicity in order to guess which language he should speak. He does so with a very high accuracy, including regional differences for Chinese people (he speaks Mandarin and Cantonese and from what I remember a few more regional dialects). He often does this without ever hearing the other person speak first, although he does sometimes overhear them speaking and butts in to chat.
>If the foreign Chinese or Korean person has assimilated (learned the language), you would hard pressed to tell.
I attend a Japanese class (in the US) with Chinese, Korean, Filipino, and Japanese students many of whom speak the language fluently. I did not have any trouble guessing except for one man who was half Korean-half Japanese. Now this isn't a topic I always bring up and it's not like I go around trying to guess peoples' ethnicity, but among friends who are learning a language together it usually comes up at some point.
Again, to reiterate: It is never 100% accurate. People are of mixed races all over the world, very few people are completely homogeneous. Recessive traits can appear, maybe they were raised in the South but by Northern parents. Maybe they're just a tall shut-in. Maybe they have a lot of makeup on. Maybe their diet was very different growing up. There's a lot more that goes into looks than just genes.
Just because most people can't play a 7-string guitar doesn't make it impossible to play a 7-string guitar.
[0] https://www.youtube.com/user/laoshu505000/videos