I wouldn't be so sure about that, especially if a member of that particular family was in on it. It's Lexicon time!Originally Posted by Molly
Phineas Nigellus Black (1847-1925)Now, it's reasonable to assume that, with the relative dates of his siblings' respective births, that Marius was born from 1916-9. If one were to extrapolate that he was born in 1916, plus postulate that Phineas Nigellus Black was Headmaster until close to his death (1925), then it is not unreasonable that Phineas would help out his son by erasing as much of Marius's existence from the Hogwarts records as he could. The reason why I believe that he would do that is because Phineas's sister, Isla, had already dishonoured the family badly enough by running off and marrying a Muggle-born. They didn't need a Squib to pop up and embarrass them further.Marius Black (disowned, dates unknown)
Son of Cygnus and Violetta (Bulstrode) Black.
3 siblings: 1 older brother Pollux, older sister Cassiopeia (1915-1992), and younger sister Dorea (1920-1977).
In terms of age of magic manifestation, I think that 5 or 6 wouldn't be unreasonable. Remember that Lily Evans already had some command over her abilities before she even went to Hogwarts, not to mention what had happened to Ariana Dumbledore at age 6. Though the Lexicon doesn't list it and my books are in storage (>.>), I am fairly certain that Neville's first vestiges of magic were at age 8. IIRC, he fell out of a window, but instead of being concerned, the relative that observed him barely stop himself from plummeting to his death was merely relieved that he wasn't a Squib. If someone has time, perhaps that could be double-checked.
So, henceforth, at around 7 or 8, assuming that Marius was born in 1916, Phineas would still be alive and perhaps even still be Headmaster. He would be able to, maybe even in his last act as Headmaster, strike Marius from the birth registers of Hogwarts and help to bury his grandson's secrets forever. If you were to go with the idea of the Black family offing their own child, then this is a means to an end in terms of anybody knowing about it.
Hope this helps.
Edit: Here is another slight addendum that might be of interest. Cygnus and Violetta's youngest child, Dorea, was noted to have married Charlus Potter. Now, by that time, I'm sure that the Potters would have already earned the reputation of 'blood traitors'. Perhaps, in order to keep their name out of any more speculation than is wrought by a child that suddenly disappeared, the Blacks allowed Dorea to marry Charlus into that family quietly and not make a big deal out of it.