Pottermore Book 2 Chamber of Secrets Pure-blood explained

Here at last we have Pottermore Information on the History and Term Pure blood! 

Personally I think a lot of this information is some what contradictory to the how the pure blood ideology appear's in Chamber of Secrets. But I always through that J.K.Rowling was holding information back in Chamber of Secrets like what Tom Riddles Diary really was e.g a Horcrux and the real purpose of the Chamber of Secrets complete with Basilisk is Salazar Slytherin 'sInternal Hogwart's security system or defence system after he left or was it shoved out by Godric Gryffindor??

The term 'pure-blood' refers to a family or individual without Muggle 

(non-magic) blood. The concept is generally associated with Salazar

 Slytherin, one of the four founders of Hogwarts School of Witchcraft

 and Wizardry, whose aversion to teaching anybody of Muggle

 parentage eventually led to a breach with his three fellow founders,

 and his resignation from the school.

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).

Magical opinion underwent something of a shift after the International

 Statute of Secrecy became effective in 1692, when the magical

 community went into voluntary hiding following persecution by

 Muggles. This was a traumatic time for witches and wizards, and

 marriages with Muggles dropped to their lowest level ever known,

 mainly because of fears that intermarriage would lead inevitably to

 discovery, and, consequently, to a serious infraction of wizarding law.*

Under such conditions of uncertainty, fear and resentment, the pure-

blood doctrine began to gain followers. As a general rule, those who 

adopted it were also those who had most strenuously opposed the

 International Statute of Secrecy, advocating instead outright war on

 the Muggles. Increasing numbers of wizards now preached that

 marriage with a Muggle did not merely risk a possible breach of the

 new Statute, but that it was shameful, unnatural and would lead to

 'contamination' of magical blood.**

As Muggle/wizard marriage had been common for centuries, those

 now self-describing as pure-bloods were unlikely to have any higher 

proportion of wizarding ancestors than those who did not. To call

 oneself a pure-blood was more accurately a declaration of political or

 social intent ('I will not marry a Muggle and I consider Muggle/wizard

 marriage reprehensible') than a statement of biological fact.

Several works of dubious scholarship, published around the early

 eighteenth century and drawing partly on the writings of Salazar

 Slytherin himself, make reference to supposed indicators of pure-

blood status, aside from the family tree. The most commonly cited 

signs were: onset of magical ability before the age of three, early 

(before aged seven) prowess on a broomstick, dislike or fear of pigs 

and those who tend them (the pig is often considered a particularly

 non-magical animal and is notoriously difficult to charm), resistance

 to common childhood illnesses, outstanding physical attractiveness

 and an aversion to Muggles observable even in the pure-blood baby,

 which supposedly shows signs of fear and disgust in their presence.

Successive studies produced by the Department of Mysteries have

 proven that these supposed hallmarks of pure-blood status have no

 basis in fact. Nevertheless, many pure-bloods continue to cite them as

 evidence of their own higher status within the wizarding community.

In the early 1930s, a 'Pure-Blood Directory' was published

 anonymously in Britain, which listed the twenty-eight truly pure-blood

 families, as judged by the unknown authority who had written the

 book***, with 'the aim of helping such families maintain the purity of

 their bloodlines'. The so-called 'Sacred Twenty-Eight' comprised the

 families of:

• Abbott

• Avery

• Black

• Bulstrode

• Burke

• Carrow

• Crouch

• Fawley

• Flint

• Gaunt

• Greengrass

• Lestrange

• Longbottom

• Macmillan

• Malfoy

• Nott

• Ollivander

• Parkinson

• Prewett

• Rosier

• Rowle

• Selwyn

• Shacklebolt

• Shafiq

• Slughorn

• Travers

• Weasley

• Yaxley

(Also as to the Pureblood family list of the sacred 28 families. Where? Oh Where? Are the Potter family, as James Potter is from a Pureblood line??? Perhaps not eh?? Oddly am a glad that the Gaunt made it as they don't appear on the Black family tree and everyone else does! )

Black Family Tree with every surname on it apart from Gaunt!

Love this Wizards of Britain Family Tree makes my heart warm as everyone is included! 

A minority of these families publicly deplored their inclusion on the

 list, declaring that their ancestors certainly included Muggles, a fact

 of which they were not ashamed. 

Most vocally indignant was the numerous Weasley

 family, which, in spite of its connections with

 almost every old wizarding family in Britain, was proud of its

 ancestral ties to many interesting Muggles. 

Their protests earned these families

 the opprobrium of advocates of the pure-blood doctrine, and the

 epithet 'blood traitor'. Meanwhile, a larger number of families were

 protesting that they were not on the pure-blood list.

* Over subsequent decades and centuries, the number of mixed

 marriages began to climb again until the healthy levels of today, and

 this has not led to widespread discovery of the hidden magical

 community. Professor Mordicus Egg, author of The Philosophy of the

 Mundane: Why the Muggles Prefer Not to Know, points out that Muggles in love

 generally do not betray their husbands or wives, and Muggles who fall

 out of love are jeered at by their own community when they assert that

 their estranged partner is a witch or wizard.

** In fact, the reverse appears to be true. Where families adhered

 consistently to the practice of marrying within a very small group of

 fellow witches and wizards, mental and physical instability and

 weakness seems to result.

*** Widely believed to be Cantankerus Nott.

Information take from Pottermore, Book 2, Chamber of  Secrets, in chapter 7

Mudbloods and Murmurs!