Existe um termo educado para "sangue ruim"?

Sim, pelo menos três

1. O mais proeminente é, é claro, Nascido-trouxa (ou "nascido-trouxa" quando usado como um adjetivo). Ron faz uma distinção explícita entre os dois termos em Câmara Secreta ¹ :

   "Mudblood's a really foul name for someone who is Muggle-born — you know, non-magic parents. There are some wizards — like Malfoy's family — who think they’re better than everyone else because they're what people call pure-blood."

   ^{Chamber of Secrets Chapter 7: Mudbloods and Murmurs"}

   E é bom o suficiente para Dumbledore, o que é bom o suficiente para mim:

   "And imagine," Dumbledore went on, "what might have happened then... The Weasleys are one of our most prominent pure-blood families. Imagine the effect on Arthur Weasley and his Muggle Protection Act, if his own daughter was discovered attacking and — killing Muggleborns... Very fortunate the diary was discovered, and Riddle's memories wiped from it. Who knows what the consequences might have been otherwise..."

   ^{Chamber of Secrets Chapter 18: Dobby's Reward"}

2. Bruxos norte-americanos têm sua própria versão de "Nascido trouxa", No-Maj-born , que é usado :

   Pure-blood families, who were well-informed through wizarding newspapers about the activities of both Puritans and Scourers, rarely left for America. This meant a far higher percentage of No-Maj-born witches and wizards in the New World than elsewhere.

   ^{Pottermore History of Magic in North America: Seventeenth Century and Beyond}

3. O artigo de Pottermore sobre "pure-bloodedness" menciona a palavra Magbob , um termo arcaico da época dos Fundadores de Hogwarts, que evidentemente caiu em desuso nos séculos intermediários (ênfase minha):

   [Salazar] Slytherin's discrimination on the basis of parentage was considered an unusual and misguided view by the majority of wizards at the time. Contemporary literature suggests that Muggle-borns were not only accepted, but often considered to be particularly gifted. They went by the affectionate name of 'Magbobs' (there has been much debate about the origin of the term, but it seems most likely to be that in such a case, magic 'bobbed up' out of nowhere).

   ^{Pottermore Pure-Blood}

¹ Hat-tip para Au101, em comentários, por sugerir que inclua esta citação

Bruxas e magos do nascimento trouxa são chamados de "nascidos-trouxas" ou "sangue-ruim". Harry disse a Horace S.

One of my best friends is Muggle-born" [HBP, Horace Slughorn]

Da mesma forma, Hermione se descreve com o termo "nascido-trouxa".

I'm Muggle-born, you see. [HBP, The Half-blood Prince]

Há muitos exemplos em que "sangue-ruim" é o insulto preferencial que Malfoy lança em Hermione.

don't touch my hand, now. I've just washed it, you see; don't want a Mudblood sliming it up. [GoF, The Weighing of the Wands]

Há um confronto entre James P. e Severus S., onde este último a chama de sangue-ruim.

I don't need help from filthy little Mudbloods like her! [OotP, Snape's Worst Memory]

James queria que Snape pedisse desculpas e ele disse a Lily que ele nunca a chamaria assim.

A maneira educada é a clara indicação de seu parentesco, "Nascido trouxa", ou Bruxa ou Mago.

Nascidos trouxas ou mestiços, por assim dizer.