The "unseen things" refer to what cannot be seen with the eye. They are "clearly visible" because people understand that they are there even though they cannot see them with their eyes. (See: [[rc://en/ta/man/translate/figs-metaphor]]; see [[rc://en/ta/man/translate/figs-activepassive]])
This expression uses the idea of the heart becoming dark to mean that they lacked understanding. AT: "their heart no longer could understand" (UDB) (See: [[rc://en/ta/man/translate/figs-idiom]])