Heilemann said:

King Arthur is King Arthur because of the blood of his father yes but that's because authority is passed through his family not any particular abilities (other than his athletic potential).

King Arthur is good at swinging a sword because he was squire to his step-brother and trained by his step-father (both trained knights).

Fantasy is fantasy but it still has to stay true to it's own rules.

King Arthur pulled the sword out of the stone because of his blood.

His royal blood gave him the mystical authority to pull the sword (only the King could do it and he is the King).

In modern parlance his magical ability to pull the sword was genetically determined.