No one could accuse the Prince of Morocco of lacking in confidence. He's fully aware of the many great qualities he possesses and the powerful effect they have on others, as he's not shy in admitting. The prince is all too aware that, in this deeply prejudiced age, Portia may judge him harshly on account of his being black. So he's keen to assure his prospective princess that his skin color is actually rather impressive to the people of Morocco. Not only do comely young maidens find it irresistibly attractive, but it scares the living daylights out of the bravest of men:
I tell thee, lady, this aspect of mine
Hath fear'd the valiant: by my love I swear
The best-regarded virgins of our clime
Have loved it too. (act 2, scene 1)
On the one hand, the prince doesn't want Portia to judge him solely on the basis of his skin color. But at the same time, he wants her to see it as an asset, something that makes him stand out from the crowd of suitors beating a path to her door.