The line "Comely but black withal," is borrowed from a verse of this song--"I am black but beautiful, oh, ye daughters of Jerusalem, as the tents of Kedar, as the curtains of Solomon." In another part of the song the reason of this blackness is given: "I am black, because the sun hath looked upon me." From which we can see that the word black only means dark, brown, tanned by the sun.
Perhaps you do not know that as late as the middle of the eighteenth century it was still the custom in England to speak of a person with black hair and eyes as "a black man"-- a custom which Charles Lamb had reason to complain of even at a later day.

The tents referred to in the text were probably tents made of camel-skin, such as the Arabs still make, and the colour of these is not black but brown.

Whether Solomon wrote the so-called song or not we do not know; but the poet refers to a legend that it was written in praise of the beauty of the dark queen who came from Sheba to visit the wisest man of the world.

Such is not, however, the opinion of modern scholars.

The composition is really dramatic, although thrown into lyrical form, and as arranged by Renan and others it becomes a beautiful little play, of which each act is a monologue.