Something, someone, or some situation is no longer as exciting, novel, or interesting as it first was, likened to (usually) a flower or fruit that is no longer in its prime and most appealing condition. It is taken from the full phrase, "the bloom is off the rose." Everyone had very high hopes for the newly elected senator, but she accomplished little in her first term, and I'm afraid the bloom is off the peach at this point. After four years, it just feels like the bloom is off the petunia in this relationship.