I'm not a biologist so I might very well be wrong about that, but it seems that one possible explanation is that the question has no meaningful answer.
Secondly, one would expect a neutral mutation to eventually be present in 50% of the population and homozygous in 25%. Or for three variations, we'd expect 1/9th of people to be O, 1/3rd A, 1/3rd B en 2/9ths AB. We've had blood types for millions of years, and no population has anything close to the distribution, which makes me believe there is some kind of evolutionary pressure.
0: https://en.wikipedia.org/wiki/Hemolytic_disease_of_the_newbo...
Also, if there is evolutionary pressure, given that we know blood groups of millions, it shouldn't be that hard to find some correlation between blood group and life expectancy, number of kids, etc. Are you aware of any?
For modeling purposes, I'll start with 50000 people, set the generation span at 20 years, and on average, each female will have 2.004 surviving children. Well, then the mutant population grows at the same rate as the general population, so remains at a tiny fraction.
So I update the model to provide a tiny advantage to the mutant by giving the mutant phenotype more surviving children per generation.
If I set up the model to start with 49998 OO, 1 AO, and 1 BO, set type O reproductive success at 2.004, type A at 2.013, type B at 2.012, and type AB at 2.013, and let it go for 2500 generations (50k years), the ending population is about 20M total, 41% O, 42% A, 11% B, and 4% AB.
That amounts to a relative advantage of only 0.45%. Obviously, the A and B mutations are very unlikely to have occurred at the same time, but it looks like both may have occurred after humans had already migrated from Asia to North and South America, as natives of South America are almost entirely type-O, and the presence of type A in North America may be explainable by Clovis migrants from Europe. So a 50ky timespan is actually a bit too long.
It seems as though there must be a selective advantage to A and B antigens, or that they be linked with an advantage, otherwise, they would not be observable as a significant population now.
[Edit:] Note that this only applies to pre-medicine. Blood type compatibility for the purposes of transfusions is definitely a survival advantage now. If current technology and politics were fixed for the next 50ky, it is likely that the "O" gene variant would become rare, and AA, BB, and AB genotypes would dominate.
It would be extremely hard to find a statistically significant indicator of increased fitness. Evolution is really good at amplifying a 0.1% advantage over countless generations to dominate a population, but to distill that signal from the noise you'd need a study with hundreds of thousands of participants.
Another problem is that the advantage from certain blood types might have only been relevant in prehistoric times, i.e. resistance to diseases that we now can easily treat and avoid. The norovirus mentioned in the article was quite dangerous back then, and still is in developing countries, but is little more than an annoyance here.
Lastly, blood type is correlated with ethnicity, which is correlated itself with life expectancy and resistance to various ailments, making it even harder to unentangle statistically.
"While group A was found to be more common in malaria cases than in normals, the reverse situation was found for group O."
and https://www.eurekalert.org/pub_releases/2015-03/ki-hbg030515...:
"It has long been known that people with blood type O are protected from dying of severe malaria." https://academic.oup.com/jid/article/195/7/1014/800831/Blood...:
"Blood Group AB Is Associated with Increased Risk for Severe Dengue Disease in Secondary Infections"
Not statistically significant? The malaria protection, in particular, seems well known (that doesn't make it true, but I find several papers in what look like respectable sources making such claims) or is severe malaria so rare that being protected against it isn't worth much?
It is clearly possible that random genetic changes happen with no impact on fitness at all. They just are.
To me, that's a shocking statement. So you're here by accident?