I had forgotten this source. It is James Hungerford's THE OLD PLANTATION (1859), which includes observations of slave songs the author heard whilst visiting a relative's plantation in southern Maryland in 1832.

First passage. The people are on a boat on a creek.

"This is getting dull," said the major, after the silence had lasted some minutes ; " Come, Charley, give us a song to enliven us a little."

In obedience to this order, Charley struck up a song; the other oarsmen answered in chorus, all timing the strokes of their oars to the measure. The song was not by any means enlivening, however, either in words or tune—as the reader will perceive. I have entitled it

SOLD OFF TO GEORGY [Chorus parts are in parentheses]

1. Farewell fellow servants, (O-ho! O-ho!)
I'm gwine way to leabe you (O-ho! O-ho!)
I'm gwine to leave de ole county (O-ho! O-ho!)
I'm sold off to Georgy! (O-ho! O-ho!)

2. Farewell, ole plantation, (Oho! Oho!)
   Farewell, de ole quarter, (Oho ! Oho!)
Un daddy, un mammy, (Oho! Oho !)
Un marster, un missus ! (Oho ! Oho!)

8. My dear wife un one chile, (Oho! Oho!)
My poor heart is breaking; (Oho ! Oho!)
No more shall I see you, (Oho ! Oho !)
Oh ! no more foreber! (Oho! Oho!)

The reader will observe that the lines of the song do not rhyme ; and it may be remarked that the negro songs—that is, such as they can compose themselves—are mostly without rhymes. When they do attempt to rhyme they frequently take more than the poetic license, being satisfied—when they can not do better—if the vowel-sounds at the ends of the lines agree.

The tone of voice in which this boat-song was sung was inexpressibly plaintive, and, bearing such a melancholy tune, and such affecting words, produced a very pathetic effect. I saw tears in the eyes of the young ladies, and could scarcely restrain my own. We heard but the three verses given (such songs are sometimes stretched out to many verses) ; for at the end of the third verse the major interrupted the song.

" Confound such lively music," he exclaimed; "it is making the girls cry, I do believe. And with such slow measure to sing to, we shall scarcely get into Weatherby's Creek tonight."
" De boat-songs is always dat way, marster," said Charley —" dat is mo' er less."
"Well, try to find something better than that," said the major; " I am sure that it is impossible for any thing to be more low-spirited in words, or tune, or manner of singing."
" Yas, marster," was Charley's answer. And the negroes sang another boat-song, but not so very sad as their first.
"Charley is right," said Miss Bettie, with a laugh; "the boat-songs are ' all that way, more or less.' I think that we had better have silence than such low-spirited music. Do you not think so, uncle?"
" Entirely," said the major. " The pathetic is well enough when there is need of stirring up our feelings of humanity, but I can see no use in creating mere low spirits."
" I like the music," said Lizzie ; " it is sometimes pleasant —if I may speak such a seeming paradox—to be made sad without any personal cause for being so. Such a state of feeling may be called ' the luxury of woe.' "
Miss Susan and I agreed with her. The negroes seemed pleased at our approval.

It looks like the 'marsters' got more than they bargained for when they requested a song!

It is printed with tune. It's in 6/8, in what you could call the major pentatonic scale (CDEGA). This gives some insight on what was being called "plaintive"! This shows that plaintive wasn't necessarily minor -- though the drop down to the sixth scale degree, presumably not characteristic of English music, lends a minor-ish touch. Blue notes may have been happening, or it may have just been something else in the "tone of voice." At least this resolves (for me) some of the disjuncture between the idea of "plaintive" and chanty melodies of today. That is, this melody is not unlike chanty melodies of today, so if this was what was being called "plaintive," then other references to "plaintive" could be to the same sort of idea.