How did Phineas Nigellus know that Hermione is a Muggle-born in the Deathly Hallows when he was blindfolded?

Question

On page 303 of The Deathly Hallows a blindfolded Phineas Nigellus is summoned from Hogwarts to visit his other picture which is in the tent with Harry, Ron and Hermione.

They ask him questions about the sword of Gryffindor after a conversation in which no names are used, he implies that he knows that Hermione is there and that she is a muggle-born.

How does he know this?

Hermione does not reveal herself to him nor do they interact much, if at all, in the previous books.

He might know that Ron is there and he knows Harry is, but even if he knows that Hermione is there how does he know that she is a muggle-born?

Answer 1

Phineas Nigellus has a portrait in the Headmaster's study at Hogwarts, and another portrait at Grimmauld Place. Hermione spent her summer and Christmas holidays in OotP in Grimmauld Place along with Harry, Ron, and the Weasley siblings. So it's highly likely he overheard a conversation where one of them mentions Hermione's muggle-born status at some time or another. While this is not mentioned in the book, Rowling doesn't cover every single conversation that occurs in Grimmauld Place at the time, so that could be his source of that information.

It could also be that, as @JanusBahsJacquet mentioned, he overheard a conversation regarding her status in the Headmaster's office, either during Dumbledore's, or more likely, during Snape's time. Let's not forget that Snape was Headmaster at a time when the Dark Lord was ruling magical Britain. Also, because of him supposedly 'murdering' Dumbledore, he would be high in Voldemort's favourite list, maybe even the highest among Death Eaters (especially since Bellatrix and Malfoy were still being punished for the debacle at the Ministry). Hermione was known to be close to Harry, and so the Death Eaters would be putting as much effort into searching for her as for Harry. So some news of her whereabouts might have been passed on to Snape, with the Death Eaters obviously referring to her as a 'Mudblood'. There is a conversation that occurs which might give slight illumination to this:

And now Snape stood again in the headmaster’s study as Phineas Nigellus came hurrying into his portrait.

“Headmaster! They are camping in the Forest of Dean! The Mudblood—”

“Do not use that word!”

“—the Granger girl, then, mentioned the place as she opened her bag and I heard her!”

_{Harry Potter and the Deathly Hallows, Chapter 33: The Prince's Tale.}

So either Snape and Phineas had discussed her before, or Phineas was present in his portrait at the Headmaster's study when Snape discussed her with another person.

Answer 2

On page 301, Phineas recognizes Harry's voice:

“Never mind where we are,” said Harry, and Phineas Nigellus froze, abandoning his attempts to peel off the painted blindfold. “Can that possibly be the voice of the elusive Mr. Potter?”

On page 302, in response to Phineas's comment about Ginny,

“Shut up about my sister,” said Ron roughly. Phineas Nigellus raised supercilious eyebrows.

Hence Phineas recognizes Ron and Harry. Now I think that he may have guessed that the trio was together.. It was known, after all, that Harry, Ron and Hermione were travelling together. That coupled with Hermione not knowing that Goblin-made armor does not need to be cleaned:

“What we really wanted to know, Professor Black, is whether anyone else has, um, taken out the sword at all? Maybe it’s been taken away for cleaning or — or something?” Phineas Nigellus paused again in his struggles to free his eyes and sniggered. “Muggle-borns,” he said. “Goblin-made armor does not require cleaning, simple girl. Goblins’ silver repels mundane dirt, imbibing only that which strengthens it.”

might have enabled him to guess who the third voice belonged to. Here, I am assuming that Phineas thought that nearly everyone from wizard families knew that Goblin-armor doesnt need cleaning, and/or Snape might have let the fact that Hermione was Muggle-born slip during one of their conversations.

Answer 3

As it has been said before, Phineas has a portrait in Grimmauld Place which is seen in The Order Of the Phoenix when they first arrive. This along with all the other portraits, and Kreacher yell and nag about 'blood-traitors and mudbloods roaming about the noble house of Blacks'

It has also been said so many times that 'Potter boy is friends with the mudblood Grainger and blood-traitor Weasley'

Truth be told, I would be more surprised if he didn't know.