Bring me the fairest creature northward born,Where Phoebus' fire scarce thaws the icicles,And let us make incision for your loveTo prove whose blood is reddest, his or mine.I tell thee, lady, this aspect of mineHath feared the valiant.
This line is spoken by the Moroccan Prince that has come to win Portia's hand in marriage. Portia's father has mandated that any suitor must correctly pick a box in order to marry Portia. The Moroccan Prince has come to give it a try. Being from Morocco means that he has much darker skin than the Venetians Portia is used to seeing. To that attitude, the prince responds:
"Mislike me not for my complexion,
The shadow'd livery of the burnish'd sun,
To whom I am a neighbour and near bred."
In other words, he is asking Portia not to dislike him because he lives close to the sun and is tan.
Then the Moroccan Prince challenges Portia to compare him to any light-skinned man. He will bleed the same color.
"Bring me the fairest creature northward born,
Where Phoebus' fire scarce thaws the icicles,
And let us make incision for your love,
To prove whose blood is reddest, his or mine."
The light-skinned man is the "creature northward born." The only guaranteed quality that the text indicates about a creature northward born is that he would have light colored skin. He would be a white guy. You could make an educated guess as to what else might be included in that description. Since it is spoken by a Moroccan, the line might reference any European, since Morocco is in Africa.