I found this letter from Frederick Douglass to Harriet Beecher Stowe, which he read to the Colored National Convention in 1853. There is only this passage specifically addressing the novel,
Dear Madam, my deep sense of the value of the services which you have already rendered my afflicted and persecuted people, by the publication of your inimitable book on the subject of slavery. That contribution to our bleeding cause, alone, involves us in a debt of gratitude which cannot be measured; and your resolution to make other exertions on our behalf excites in me emotions and sentiments, which I scarcely need try to give forth in words. Suffice it to say, that I believe you have the blessings of your enslaved countrymen and countrywomen; and the still higher reward which comes to the soul in the smiles of our merciful Heavenly father, whose ear is ever open to the cries of the oppressed.
I wonder if that made Frederick Douglass an "Uncle Tom?"