How was the Bible used to justify slavery in the south?

First of all, The Bible nowhere condemns slavery explicitly and several Biblical figures like e.g. Abraham had slaves without this raising any negative comment in the text.

Secondly, in the story of Noah in the book Genesis, Noah cursed his grandson Canaan (and supposedly also Canaan's offspring) 'to be the lowest of the servants to his brethern'. Although the witer(s) of the book Genesis probably meant this only as an excuse for the Israelites to subject the original, indiginous people of Canaan (Palestine), later Jews, Arabs and Christians saw this as an excuse to enslave these supposed descendants. Why blacks? The black skin was seen by them as a sort of visible proof that THEY must be the descendants of the man who was sentenced to be a slave to others.