Seneca, On Anger
2.36.2 (tr. John W. Basore):
If the soul could be shown, if it were in some substance through which it might shine, its black and mottled, inflamed, distorted and swollen appearance would confound us as we gazed upon it.
animus si ostendi et si in ulla materia perlucere posset, intuentis confunderet ater maculosusque et aestuans et distortus et tumidus.
American Catholics of my generation will remember the illustration of the soul in the Baltimore Catechism, as a milk bottle, with black spots representing sins.