I finally found it!
I have been searching, since I was a freshman in High School for a poem. I didn't know the title. I didn't know the author. All I knew was that it touched me profoundly and the first and last lines had been running through my head for, um, 13 years. I knew it was a black writer, as I had found it in a book of American black poets at the school I was going to at the time. But after I moved, there was no way I could return to the library to try and find it again! I thought it was from the Civil War era, but now I know it was much later than that and that the writer was a Jamaican man well known in Harlem. Here it is, I hope it touches you the way it touched me then and still does now.
By Claude McKay
I must not gaze at them although
Your eyes are dawning day;
I must not watch you as you go
Your sun-illumined way;
I hear but must not ever heed
The facinating note,
Which fluting like a river reed,
Comes from your trembling throat;
I must not see upon your face
Love's softly glowing spark;
For there's the barrier of race
You're fair and I am dark.