From RHJunior Comics Wiki
Mageblind people can neither sense nor manipulate lux. While this condition holds true for most humans, it is an exception for the Rac Cona Daimh. Affected individuals wear a white bead to indicate their handicap. A notable example is Brother Linnaeus, the Sojourner priest serving among the Gragum in the Dire Swamp.
Judging from the lack of fear from the other Rac Cona Daimh, it appears that the "mageblind" trait isn't passed on genetically in a manner that promises that any child of a mageblind suffers from the condition as well but is a random failure in part of the physiology of them. Making it a randomly occurring trait such as dwarfism or other such traits like humans.
Though it has not been explored in this story, noting that the Racconans are not immune from faults like humans, one can assume that there is some distaste towards mageblinds, either in a worry in the more ignorant people that it can infect others or perhaps a sign of weakness in the race.
Though we can see in the story that people like Linnaeus can use this "disability" to ease the worry of other races over the powers the Racconnans have and make it easier to speak to them. People are more willing to listen and hear if they aren't scared or plotting in the back of their minds.