Some thoughts on the old magical families and bloodlines, and where muggleborns come from.
"Because I hated the whole lot of them: my parents, with their pure-blood mania, convinced that to be a Black made you practically royal..." - Sirius Black, Harry Potter and the Order of the Phoenix
Why did the Blacks think of themselves as "practically royal"? If they can trace their ancestry far enough, to the beginnings of Britain's magical culture, perhaps they had reason to think so.
We've seen some of Britain's magical community's roots. Merlin is such an important figure in their history that not only is one of their highest honors named for him, the Order of Merlin, but his name is used in common expressions like "Merlin's beard" in much the same way muggles say "My God." We also see other prominent Celtic mythological figures honored in the series such as Morgan le Fey and Queen Maeve. The roots of the British magical community lie in the figures of Celtic myth.
Looking at the mythology JKR is using in this series, it's all about family, and royalty.
Merlin, King Arthur, and the Grail
The most famous of the Celtic myths are those of King Arthur and his Knights. Arthur was the British High King who fought the Romans and later held off the Saxons. Arthur's, and Merlin's, family was descended from the Irish High Kings and Queens. Arthur's half sister was Morgan le Fey, who ruled in Avalon, one of the magical places of the Sidhe, the Fairy Folk. This is where Arthur's sword, Excalibur, was forged, and is Arthur's final resting place. But, some say, he just sleeps... According to the legend, Jesus visited there, and Joseph of Arimathea would later place the Holy Grail there, and marry into the family.
Now, some may be wondering how Jesus and Christianity would be right at home with all these powerful witches. The old Celtic form of Christianity was very different from what we've known, since, and reflected the Celtic culture. Magic wasn't seen as a bad thing, and women were considered equal to men, holding positions of power, High Priestesses, and often leading armies into battle. Their form of Christianity was Gnostic, and blended seamlessly into their culture and traditions. This was how things were until the Saxons finally conquered the land.
For more on Avalon, see here.
http://www.uidaho.edu/student_orgs/arthurian_legend/england/sites/avalon.htm The old Celtic tales were revived by the Courts of Love of Eleanor of Aquitaine and her daughter, Marie. There is, reportedly, a lot of alchemical and numerological symbolism in these versions of the tales. But, Eleanor, at a time when most didn't live past their 30s, lived into her 80s, and magical folk are supposed to have longer lifespans...
There were prophesies attributed to Merlin, including an interesting one that France would be ruined by a woman, and restored by a maid from the borders of Lorraine, where, of course, Joan of Arc would be from. Many Scots were in her army, and she was of Celtic descent. Her mother, Isabelle Romee, in a time and place ravaged by both the war and the Black Plague, lived into her 80s.
The Family
Arthur was the final great Celtic myth, but hardly the only one. The mythology of the Celtic Kings, Queens, and Heroes is a long and rich one. This is the basis and origin of the British Wizarding world of the Harry Potter books. Joseph of Arimathea was descended from King Solomon, so his marrying into the Celtic royal family brought the House of David into the bloodline, but it was an impressive lineage, already. We start with the Tuatha De Danaan, who ruled Ireland before the Milesian Conquest.
The Tuatha De Danaan, or the People of Danu, originated in Ancient Greece, children of the Goddess Danu and the Cosmic Snake Ophion. They arrived on what would be known as Ireland in a great myst, coming from the sky, where they fought and defeated the giants that had previously ruled the land. They were known for their beauty, great magic, and wisdom. They brought with them the Lia Fail, Stone of Destiny, on which British monarchs are crowned to this day.
Eventually, they were conquered by the Milesians. Most retreated into magical places, the most famous of which is Avalon, and became the Sidhe, the Fairy Folk, and were worshiped as gods by the Celts. Ireland was named for their last Queen, Eire. The first Milesian High King of Ireland, Crimthan MacNair, married one of the Tuatha de Danaan, keeping the ancient blood in the new royal family.
For more on the Tuatha De Danaan, see here.
http://www.likesbooks.com/tuatha.htmlhttp://www.shee-eire.com/Magic&Mythology/Races/Tuatha-De-Danann/Page1.htm The Sidhe are described as blonde, fair skinned, and have prominent, striking, eyes. Sounds a lot like Luna Lovegood, doesn't it? She has an old magic feel to her, her eyes are large and moonlike, and never seem to blink "like a normal human's" (OotP). She unnerves people when they first meet her. Mr. Ollivander's eyes are described exactly the same way, and we know his family is old. The Ollivanders began their wand making business 2400 years ago, right about the time the Irish captured and named Scotland.
The Egyptian Princess
The Milesians weren't without magical blood, themselves. Scotland is named for Scota, daughter of the Pharaoh who was the wife of the leader of the Milesian invaders of Ireland, and quite the military commander, herself. The Egyptian Royal family was, as we know, quite magical. For more on Scota, see here.
http://www.angelfire.com/home/thefaery6/Scota.html So, with the magical lineages of the Tuatha De Danaan, the Egyptians, and Solomon in their ancestry, the old magical families can claim to be "practically royal", as the royal families of Europe trace their ancestry to those very same Irish High Kings and Queens. But, do they really have a better claim on the magical heritage than the muggleborns?
Where Do Muggleborns Come From?
Andromeda Black was considered a "blood traitor" for marrying muggleborn wizard Ted Tonks. The Tonkses first appeared in history in Nottingham, where they were made Lords of the Manor after the Norman Conquest. Like the pureblood magical families, all the muggle nobility was eventually related. So, any muggleborn with that ancestry is descended from the Celtic magical families.
The Tonks Coat of Arms
http://www.houseofnames.com/xq/asp.c/qx/tonks-coat-arms.htm It has been statistically proven that everyone of European descent is descended from Charlemagne. Charlemagne, in turn, was descended from the Irish High Kings and Queens. Going back further, it's statistically proven that pretty much everyone in the world has Egyptian or Asian royalty as ancestors. So, literally everyone in the world has magical blood. There's not only no true purebloods, but there are no true muggles left in the world.
The Riddle Coat of Arms
http://www.jimcal.com/images/SEAL2.JPG Bears a striking resemblance to the Coat of Arms on the Black Family tapestry, doesn't it? Some common ties, perhaps? Tom certainly resembles his father far more than his mother's family. Perhaps mixing with the Riddle blood is just what the Gaunt blood needed to regain it's old potency, much as mixing Tonks blood into the Blacks produced a metamorphmagus?
The Queen's Tomb
Finally, we have a Right of Passage that every magical child in Britain must take. To go to Hogwarts to learn the ways of magic, students are required to take the Hogwarts Express from Kings Cross Station in London. But, why? Why make this pilgrimage to the heart of muggle Britain, to walk among the muggles, to use a muggle transportation system? The Statutes of Secrecy had to be relaxed just to do this, as it required the cooperation of Kings Cross's builders and maintainers. So, what's so special about King's Cross?
Well, it's said to be the final resting place of Queen Boadicea, Celtic warrior Queen who led an uprising against the Romans. The passageway for children beginning their journey to learn magic is the Tomb of one of their prominent ancestors.
For more on Queen Boadicea, see here.
http://en.wikipedia.org/wiki/Boudica