What was the one thing about John's death that Joan was unable to share with her friends as noted in the memoir, The Year of Magical Thinking?

In the opening chapters of the book, the author writes that there was one fact about John's death that she couldn't bring herself to tell her friends, no matter how many times she told the story of John's death—the fact that John's blood had spilled on the floor. This is the first of several important images relating to the author's inability to accept the reality of her husband's death.