Bermuda of the Vindice is a Black.
And he was an Heir, specifically the one "murdered'' in the 18th century that was mentioned in chapter 39. After becoming a Vindice, he lost his position as heir. The Blacks presumed he was assassinated since his brother obtaining the Heirs Magic indicated as such. He still has some family magic, just not the Lords or Heirs magic. This is why he can create the Flames of Night. Skull de Mort is Death's son.
It is right in his name. And Death will be pissed off when he finds out his son has been Cursed. Tyrant of the Varia is Death's son
He has not aged in living memory and no one knows who he is and where he came from. Also, a powerful Sky is far more likely to be half Titan. Xanxus and Dorea's next child, the Heir Zabini will be a Sky.
Cassie and Marius are not due to the strong Black predisposition in their genetic makeup. Hector, who has strong Potter genes is a Sky. Daniela is suspected to be a Sky. Both Dorea and Xanxus are very strong Skies, and the kid will heavily favor Xanxus' side of genetics.- Do remember that the Zabini Family Magic is strongly biased towards Storm Flames, so Xanxus and Dorea's upcoming son could very well be a pure Storm.
Timoteo Vongola is a bastard himself.
Daniela Vongola had some rather suggestive thoughts that indicate that she was a False Widow. And her son was born out of wedlock and she could not pretend to have a husband for long enough to birth a few more children.- Bolt From The Blue uses many characters and backstories seen in this story, and Daniela explicitly conducted a charade of courting and wedding because a husband would have had power over her, so she could have done the same here. Also, Theo noted Don Nonno quite resembled his mother's Rain Guardian...
The psychotic breakdown of Prince Rasiel of Prussia was arranged by his parents.
Ramiel aka Belphegor does not have the genetics to wield a wand and is the identical twin of Rasiel. Both of them are raised in a separate estate, their parents have only visited a few times in seven years. Their proven witch daughter Suriel is raised separately, to the point were Bel has never even met her. Most magical authorities agree that wizarding magic typically appears prior to the age of seven. It is possible that the Prussian royals have a method that identifies squibs prior to that age which explains why their twin sons were raised away from their parents and their sister. Then at the age of seven Rasiel has a mental breakdown and attempts to murder his twin with 'magic' which he should not be able to use due to being a squib. It is quite possible that he was placed under the Imperius curse or some other form of mind control which caused his Storm Flames to activate. This way the royal family of Prussia got rid of one squib and had legal justification to disown the other.