The Niles original lyrics do not seem to be available on a website. I can only suggest that someone who has the book issued by his music publisher please post the words here.
G. Schirmer, publisher. "The Songs of John Jacob Niles." Piano and vocal. $US 15.00

I do recall that his version, on an old RCA Red Seal (1940s) set of folk, was brief, but I can't recall any of the words now. In the following note, Niles refers to his 1958 recording- same words??

""Black is the Color of My True Loves Hair" as sung here was composed between 1916 and 1921. I had come home from eastern Kentucky singing the song to an entirely different tune- a tune not unlike the public domain material employed even today. My father liked the lyrics but thought the tune was downright terrible. So I wrote myself a new tune, ending in a nice modal manner. My composition has since been discovered by many an aspiring folk-singer." John Jacob Niles. Music

Posts above show that there was an older song with a different tune. The Niles version and its copies have taken over, however.

I like the Ellis Peters version from her murder mystery:
Black, black, black is the color of my true-loves heart!
His tongue is like a poisoned dart,
The coldest eyes and the lewdest hands...