That question has more interpretations than we will be able to cover.
I think we need to think first of blood in the sense of heritage,
whether as genetics, genealogy, or metaphors.
Even as heritage, blood is ambiguous. I think it has previously been
established in this forum that Faerie Blood does not necessarily refer
to genealogical heritage or birth rights. We had a young character born
in our saga with faerie blood, simply because he was conceived at a
faerie festival with an ample supply of faerie wine, by otherwise non-faerie
parents.
With this interpretation, blood can infer to any kind of mystic legacy,
which can be transmitted in any number of ways, limited only by our
collective imagination and suspense of disbelief. Blue, royal, or noble
blood possibly has some mystic quality too, even when some people still
get elevated from commoner. That would not be more or less controversial
than the divine MR that kings enjoy.
When it comes to the Gift, I find canon conveniently ambiguous. The Gift
is not heriditary. Players cannot secure apprentices and grow the Order
through any systematic breeding programme. OTOH, it seems that genetic
heritage does give an increased probablity of getting the Gift, lest Mercere
would have had even greater trouble finding true blooded apprentices.
Now, is any of this related to some any bodily fluid (humour) known as blood?
Certainly, that must have been the mythic belief, lest the word would
never have come to refer to genealogy. However, if it does, it seems
that blood quality is not purely genetic. If a remore cousin of the late
king rises to the throne, and receives the Divine MR, would that mean that
his blood is becoming more potent, bluer so to speak? I suppose that works
in mythic narrative. Maybe faeries, when they directly or indirectly bestow
faerie blood on someone, really bestow faerie powers in their blood. Why
not?
The same would be the case for the Gift. If Faerie Blood works like this,
we could also have a Gift in the blood, without its breeding true. It works
in a mythic narrative.
I am sure, irrespective of the actually nature of the Gift, there have been
Hermetic doing research on blood, to trace the nature of the Gift and other
powers. There may not have been many, since it would be ethically questionable,
just like other areas of necromancy, but some have tried and either failed or
kept their secrets. That works in the narrative too. Only a few rare sagas
would want to have any research notes surface from such projects.