The problem with the nohr dark blood is that you already have way too may dark mages and others that can you use tomes. If your Kamui isnt good in magic, I saw no real reason to even use the tomes when i promoted. End game I had 2-3 tome users. With the Light blood, staves are at least a little more useful since unless you have 5 healers on a team, you cant go wrong with a unit just topping off your hp.
And yes, Ive gone through both pathes. Both have their uses and are great promotions(dark blood looks cooler) but..yeah #opinions