From what I've read, the Jordan River is sung about so much in black spirituals because it symbolized escaping across the river to freedom. The song On Jordan's Stormy Banks I Stand seems to make that connection clear:

"On Jordan's stormy banks I stand
And cast a wishful eye
To Canaan's fair and happy land
Where my possessions lie

I am bound for the promised land
I am bound for the promised land
Oh, who will come and go with me?
I am bound for the promised land

Jerry