The black bag is the image through which Tolstoy shows the passage of death seen from within: Ivan Ilyich feels himself pushed, with pain, into a narrow, deep bag, unable to get through.
It appears for the first time in chapter IX, under the effect of opium: he feels himself stuffed into a narrow, deep black bag, and pushed deeper and deeper without being able to pass. He wakes with his legs on Gerasim's shoulders and weeps. In chapter XII, during the three days of screaming, he struggles in this bag, and the narrator says that the torment lies in not being able to pass through.
What prevents him, the narrator says, is the recognition that his life was good. The same defense that accompanied him through the earlier chapters becomes a physical obstacle: the bag opens only when he lets go of the justification of his own life. Then a force pushes him in the chest, he falls into the hole, and at the bottom something shines.
The text itself offers a comparison: it is like a railway carriage, when one thinks one is going forward and is going backward, until suddenly one discovers the true direction. The bag can be read as the correct life itself, which protected him and now imprisons him.