Mary, I found this little rhyme. Its variations could explain how "razor soup" could get two meanings. Thomas W. Talley, a black Chemistry professor at Fisk Univ., collected Negro Folk Rhymes and published them in 1922.
DRINKING RAZOR SOUP

He's been drinkin' razzer soup;
Dat sharp Nigger, black like ink.
If he don't watch dat tongue o' his,
Somebody'll hurt 'im 'for' he think.

He cain't drive de pigeons t' roost,
Dough he talk so big an' smart.
Hain't got de sense to tole 'em in.
Cain't more 'an drive dat ole mule chyart.

Most of the rhymes were collected before 1920, so the song and talk of Southern Negroes, 1900-1940, are captured in his book. The "N" word was used frequently by them. It had several senses, difficult for someone like me who is not Black to understand. It meant "us together against them," it meant "low-down no-'counts," (unlike the right-living speaker), it was a lament about insignificance and status, and, depending on the situation, probably had other shades of meaning as well. Perhaps only the Negro convicts on the large prison farms use it regularly now.
The book has been enlarged with many more rhymes from Talley's notebooks and published by The University of Tennessee in 1991, "Negro Folk Rhymes." Discussions by Talley and C. K. Wolfe contribute to a most interesting book.