Orson Pratt describes the plates as having a black, hard stain, allowing the letters to be more legible and easier to read.
Orson Pratt, "Evidences of the Bible and Book of Mormon Compared," January 2, 1859, Journal of Discourses, 26 vols. (Liverpool: Amasa Lyman, 1860), 7:30-31
Eight other witnesses testify that Joseph Smith showed them the plates, and that they saw the engravings upon them, and that they had the appearance of ancient work and curious workmanship. They describe these plates as being about the thickness of common tin, about eight inches in length, and from six to seven in breadth. Upon each side of the leaves of these plates there were fine engravings, which were stained with a black, hard stain, so as to make the letters more legible and easier to be read. Through the back of the plates were three rings, which held them together, and through which a rod might easily be passed, serving as a greater convenience for carrying them; the construction and form of the plates being similar to the gold, brass, and lead plates of the ancient Jews in Palestine.