I understand your perception, but that's what you get in a straight mulatto mix. Bob Marley was a mulatto. Look at him.
Especially when his father was so very dark to begin with. His father and mother probably made the appearance of an average black American family that has been here for 200 years.
A very simple breakdown of Natural Born citizen.
http://www.youtube.com/watch?v=GqH7rSHcvgU
The British Nationality Act 1948
Section II
Citizenship of the United Kingdom and Colonies.
Citizenship by birth or descent.
5.(1) Subject to the provisions of this section, a person born after the commencement of this Act shall be a citizen of the United Kingdom and Colonies by descent if his father is a citizen of the United Kingdom and Colonies at the time of the birth:
http://www.uniset.ca/naty/BNA1948.htm
He was also straight.