The question regarding Domithilde LeBlanc who married Joseph Cormier is a lot more complicated than it appears at first glance, as I am sure you realize, or you would not have referred it to me in the first place. As you suggest, there is no original documentation to show that Jean LeBlanc and Claire Bourque had a daughter named Domithilde. There was no daughter by that name in Jean's household at the time of the 1851 census, although Joseph Cormier's wife ought to appear in that census, considering that she was married only three years later.
Domithilde and Joseph's marriage record does not say who their parents were, although someone later marked in the register that Joseph was a son of Placide Cormier and Marguerite Gaudet, which appears to be correct, and that Domithilde was a daughter of "Jean à Jean à André." This is where the idea originates that Domithilde was the daughter of a Jean LeBlanc. But Claire Bourque's husband wasn't Jean à Jean à André, but rather Jean à Jean à Jean-André, so the match is not quite right anyway.
I checked the 1861 and 1871 censuses. Joseph Cormier and Domithilde appear in both of these as a married couple. Domithilde was twenty-five years old in 1861, and thirty-five years old in 1871. The consistent result in these two enumerations is encouraging. She obviously was born around 1836. And she should appear somewhere in the 1851 census as a girl aged about fifteen.
The index to the Memramcook registers shows two Domithilde LeBlancs who were born in 1836. One was the daughter of a Pierre LeBlanc. It can be shown that this Domithilde married someone other than Joseph Cormier. That leaves Amand LeBlanc's daughter, who was born and baptized Nov. 17, 1836. She does appear in the 1851 census, aged fourteen. I have found no information that would rule her out. And what's more, I have found a connection which could explain why someone might have thought that this Domithilde was the daughter of Jean LeBlanc and Claire Bourque. Amand LeBlanc was first married to Claire Bourque's sister Marie. Domithilde was Amand and Marie's first child. So the latter-day note in Domithilde's marriage record in the parish register of Memramcook might simply reflect confusion between the families of the two Bourque sisters who married LeBlancs. Incidentally, if you look into the Cahier with the genealogies of the 37 families, you will see that I had been unable to identify Claire Bourque's parents in 1994 (p. 162). I have since learned that they were Laurent Bourque and Apollonie Melanson (pp. 100-101), which does make Claire a sister of Amand LeBlanc's wife. You will also see that a third Bourque sister married yet another LeBlanc. This marriage only occurred in 1840, however, so there is no chance that Domithilde could have been the third couple's child.
Unfortunately, I have found nothing that really would confirm this identification of Domithilde. I even looked up the baptismal records of Domithilde's first six children, to see who their godparents were. The godmothers included Cresence LeBlanc, Séraphine LeBlanc, Osite LeBlanc, and Euphémie LeBlanc. The first of these four was actually Joseph Cormier's half-sister. (Marguerite Gaudet had married secondly Pierre LeBlanc.) I have not been able to determine who the other three were; none of their names coincides with any of the names of the children of Amand LeBlanc and Marie Bourque, insofar as I can see. I do note, however, that only one of these first six children had a double name. Domithilde's third daughter was baptized Marie-Marguerite. As you may know, there was a tendency to give such double names only where doing so would honour both grandmothers, or both grandfathers. Consequently, Marie-Marguerite might have been named for Marie Bourque and Marguerite Gaudet. On the other hand, these two given names are so common that such a conclusion can be no more than tenuous at best. And while Joseph and Domithilde did name their first son Placide, obviously after Joseph's deceased father, it would have been far more convenient for our purposes if they had given that boy the name Amand, too.
If Amand LeBlanc's daughter Domithilde should prove to be the right one, then your correspondent must be rather closely related to our former governor general, Roméo LeBlanc, because Roméo's mother's father was Amand LeBlanc and Marie Bourque's son Jude.
The irregularity of her situation is why we had a relatively hard time determining who Domithilde LeBlanc was.
It was easy to find out that Joseph Cormier's wife was not the former governor general's grandaunt. That Domithilde, as you will recall, was fourteen years old at the time of the 1851 census. I checked the 1861 census, and soon learned that she was twenty-four in that listing and still unmarried and living in her father and mother's household. The Domithilde you have been seeking meanwhile was married in 1854.
I then looked a little more closely at the location of Joseph Cormier and Domithilde LeBlanc in the 1861 and 1871 censuses, relative to other families. In the transcription of the 1861 census they are shown in household 0012. Jean LeBlanc and Claire Bourque are in household 0011, which one may presume to have been right next door. In 1871 the Cormiers are in household 180 of section 4, while Jean LeBlanc and his second wife, Marie Saulnier, are in household 178. From these listings it is quite clear that there was a close connection between Jean LeBlanc and Domithilde.
But he was not her father. This is shown by the 1851 census. As has already been noted, Domithilde does not appear in Jean's household in that enumeration. So where was she? I first thought that at age fifteen she might have gone out to work for someone else, and on that premise I began looking through the "White" and "LeBlanc" listings to see whether I could find her. I did not have to look for long. Domithilde was listed on the same page as Jean LeBlanc and his family in 1851. She was quite probably once again just next door. But she was not a servant in another home. The census states that she was living with her mother.
I should mention here that, as you are probably aware, most of the LeBlancs in the civic parish of Dorchester in 1851 are listed as Whites. There are a few LeBlancs. Most of the latter appear on pages 60 and 61. These are all members of the family of Jean à Jean-André LeBlanc. One household consists of three females. This is where Domithilde was living. The head of the house was "Margaret LeBlanc," a "widow," aged thirty-seven. Then came Domithilde, "her daughter," aged sixteen. And the third female was "Mary LeBlanc," another widow, aged seventy. The last of these was Marie Comeau, Jean à Jean-André LeBlanc's widow. She was actually about seventy-four. Marguerite was her daughter, and Domithilde was Marguerite's daughter. As is noted in the LeBlanc genealogy I prepared in 1994, Placide Gaudet indicates that Marguerite married Norman DesBarres. It could be that she was in fact DesBarres's widow at the time of this census. But it does not appear that Domithilde could have been DesBarres's child, at least not his legitimate child, because Domithilde was always known by her mother's maiden name, which suggests that she must have been born before her mother married. In any event, the census makes it clear that Domithilde was the daughter of Marguerite LeBlanc, and that her mother was the daughter of Jean LeBlanc and Marie Comeau. This conclusion is consistent with the dispensation for fourth degree consanguinity that Domithilde and Joseph Cormier received when they married. Joseph's mother was Marguerite Gaudet, his mother's mother was Marguerite Babineau, and her mother was Marguerite Léger. This Marguerite Léger was a sister of Marie Comeau's mother, Anastasie Léger.
So that is the end of part of the mystery concerning Domithilde. Unfortunately, as is usual in such cases, there does not appear to be any way to find out who her father was. I am sorry that I cannot offer your correspondent anything better in this regard.