Niles himself said, à propos his 1958 recording of the song:

"Black is The Color of My True Love's Hair as sung here was composed between 1916 and 1921.  I had come home from eastern Kentucky, singing this 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 it in a nice modal manner.  My composition has since been "discovered" by many an aspiring folk-singer".

Between 1916 and 1918, Cecil Sharp got a variant from Mrs. Lizzie Roberts of Hot Springs, N.C ., which is in the DT:  BLACK IS THE COLOUR (2)  He himself considered it to be of English origin; the verses are all "floaters" which appear in other songs, too, in Britain and Ireland as well as America, so I doubt if it's a translation from any other language.   The melody is undoubtedly a very close relative of the one which Niles said that he composed a few years later.  Clearly, Niles' "new" tune was no more than a modification of an existing, traditional one; the same is of course true of his text.

The DT text at  BLACK IS THE COLOR OF MY TRUE LOVE'S HAIR  is described as "From Sharp", but no useful details are given.  "Clyde" appears in both; I don't have Niles' text, and most references on the net are confused and unattributed, but those from Sharp, at least, pre-date his.  I believe I've seen "Troublesome" (glossed as "the name of a town") substituted for Clyde somewhere.

You might like to have a look at these two at the  Max Hunter Collection:

Black, Black, Was The Color Of My True Loves Hair  As sung by May Kennedy-McCord in Springfield, Missouri on September, 23, 1958.  This is a variant of A Sailor's Life, but compare:

Black is the Color of My True Loves Hair  As sung by Mrs. "Bobbie" Barnes, Eureka Springs, Arkansas on June 21, 1958.  The editors categorise this one-verse fragment as belonging to the same group, possibly on the strength of the correspondence of some lines in the foregoing.  The tune, however, shows a distinct resemblance to the Niles one.  Of course, Mrs. Barnes may have learnt it from him, but that still wouldn't explain Mrs. Roberts.  It might be worth comparing some more Sailor's Life tunes.

Willie Clancy is supposed to have picked up Dark is the Colour from an American singer in Warsaw in the late 1950's, and started playing it himself; that may be as Irish as it ever was.

Malcolm