The entertaining aspect of sparsely studied questions is that you'll find the evidence to be all over the map. No study can be considered in isolation, as many are in some way flawed - the right way to read science is to look at the flow and interaction of many groups and their research results. But if only a few researchers are publishing on a given topic, there's no real way to balance their work in a broader context. The only thing to do is wait.
We might look at blood type and longevity in this context, as a paper on this topic recently caught my eye. I don't recall reading anything on blood type and life expectancy in the past few years at least, so I know it's not a mainstream concern or a matter of common knowledge in the research community.
To assess the observation that blood type B might be a marker for longevity, we reviewed the records and determined the ABO blood types of all patients who died in our hospital in 2004. ... In our patient population, the percentage of patients with group B blood declines with age. The survival curve in group B was worse than that in groups A, O, and AB. These findings suggest that in our patient population, blood group B is not a marker for longevity but may be a marker for earlier death.
Which seemed interesting enough to merit a very brief search of the literature to see what else was written on the topic. That turned up this paper from 2004:
The aim of the present study was to investigate the association between blood groups and life expectancy. We compared frequencies of ABO blood group in 269 centenarians (persons over 100 years) living in Tokyo and those in regionally matched controls ... Our findings suggest that blood type B might be associated with exceptional longevity. Responsible mechanisms need to be investigated.
This is par for the course in biology. They might both be right, or they might both be wrong. Either way, both centenarian genetics and the biochemical details of the aging immune system are presently under investigation by a fair number of researchers. If there is a good answer to be discovered over the next decade or so as to whether - and how - blood type might influence longevity in various populations, it will probably come as a side-effect of these areas of research.
As is true of a range of the scientific research I reference here at Fight Aging!, that this is interesting doesn't mean it has any great relevance for the future of our longevity. Either SENS or SENS-like repair biotechnologies are developed over the next three to four decades, or we'll all grow old and die. Oh, we'll probably live slightly longer than our parents in that scenario, thanks to drugs that manipulate metabolism to slow the rate at which biological damage accumulates, but that would be a pretty poor outcome in comparison to real, working rejuvenation medicine to make people young for as long as they want.
One effect of developing biotechnology to repair the biochemical damage of aging is that the quirks of our biology will become irrelevant. No one will scrabble for a little statistical gain here or there, or worry about blood type, exercise, calorie restriction, their genes, or whether they have a good mitochondrial haplotype. It will be a great leveling of the hand dealt to us at birth, as well as eliminating the suffering and frailty of of aging.
So interesting research is interesting, especially when it illuminates truths about the way in which the research process proceeds in sparsely populated reaches of the life sciences - but don't mistake that for relevance.