Huck pretends to be someone else and tells a fabricated story about why his steamboat took so long to reach its destination. He explains a cylinder blew up, causing the delay. Mrs. Phelps wants to know if anyone was hurt. Huck explains that no one was hurt and then goes on to say that a black person was killed. Mrs. Phelps is happy to hear this because she knows having a cylinder explode can be very dangerous.
The fact that Mrs. Phelps does not view the death of an African-American as a tragedy is ironic, because we as readers view it as a loss of human life while Mrs. Phelps does not.