would douglass have agreed with lincoln's characterization of slavery as a national sin?

Answer 1

If you mean Frederick Douglass, the African-American abolitionist leader, I have to believe the answer would be yes.
http://www.pbs.org/wgbh/aia/part4/4p1539.html
http://en.wikipedia.org/wiki/Frederick_Douglass

If you are talking about Stephen Douglas (one S), the Senator and Lincoln's political opponent, I would have to say no.
http://www.spartacus.schoolnet.co.uk/USAdouglas.htm
http://en.wikipedia.org/wiki/Stephen_Douglas