I think you can be born with many predispositions that may lead a person to feel like they're a natural witch. For example, I have been enamoured with witches ever since I was a child. Before I could write I would draw them, before I started school I'd spend sunny days sitting in the garden plucking leaves for my potions and putting them in shampoo bottles (I told my mum, trust me, your pantene pro-v will be improved by all the twigs, special bits of mud, glitter and occasional dead insects.)
I was inspired, and while my brother did none of these things despite being exposed to the same influences I was, I?ve never been able to let go of witchcraft and do consider it an innate part of me.
What you think about natural witches in the way I think you mean (bloodlines, powerful gifts and so on) all depends on your definition of what a witch is in the first place! Or even more fundamentally, what you think it means to be human and what kind of universe we live in.
Anyone who has spent much time with children will see that some, for example, are more inclined to draw rather than play music, some are more inclined to dance than play sports, and some are more inclined to read a book than to talk. All kinds of things factor into that though. A natural witch could be someone who grew up (for whatever reason) with inclinations compatible with witchcraft, but to say someone is born knowing what rituals to do seems to me more fiction than reality.
A child might be a born dancer, but they weren't born knowing how to tango. Mozart always had a flair for music but he had to be taught the scales by someone else like every other musician before and after him. Doing rituals generally might be an impulse wired into the brain (childhood psychologists like Bruce Hood talk a lot about this) but knowledge of specific rituals? I have my doubts.
I know you know that when my family tell me I'm "a natural at cooking" you mean it in a different way to being a natural witch. However, I think the idea of a natural witch was created to add to the mystique around witchcraft. Traditionally witches aren't like normal people after all. They're occult: their knowledge is hidden, and so (people thought) maybe the ways that someone becomes a witch is a hidden kind of magic too. Witchcraft was feared, and the randomness around witchcraft was feared too. Who might succumb to it? Who might turn out to be one? Eventually fear seems to have turned into romanticism. It?s very sexy, to be born a witch or are somehow chosen, either by blood or by bequest powers.
This is why no one talks of natural Christians, Hindus, Buddhists or Muslims: these are ?normal people? but witches are almost always given a supernatural twist.
I don?t think anyone is immune to stereotypes or definitions of witchcraft in our society either (you could argue well that natural witches and Native American bloodlines are stereotypes, for example.) I grew up loving witches, not because I loved how evil and gross they were supposed to be but because the witches I was exposed to were all like me in someway: The Worst Witch, Witchchild, and (more recently) Harry Potter. For every Wicked Witch of the West, there is a Glenda the Good.
The ideas about witchcraft in society are complex, and these ideas are always being reconsidered and re-described as we talk.
So, as said above all of us are born naturally to be able to cast magic, and as said we can keep learning and practising.
It is true that there are people who recieve more gift in magic stuff, but if you practise you can be excellent!