"Ballad for a Blue Lady" is a weak episode even though unlike most weak episodes it actually stars Angela Lansbury. Normally, the weaker ones are the so-called 'bookend' shows where Lansbury just introduces the show and the mystery solving is left to some guest star.
The story begins with Jessica's friend, the famous country music singer Patty Sue Diamond (Florence Henderson) inviting her to Nashville for a concert. However, soon Jessica learns that there's another reason Patty Sue invited her...and what follows really doesn't make a lot of sense. I could say more about how the killer's actions didn't make any sense....as it would spoil the suspense. But it just doesn't make sense.
In addition to the plot-holed plot, the episode also is weak since Florence Henderson's country accent seemed TOO strong...and this twang also negatively affected her singing (which is usually much better). My score of 6 is probably a bit generous for this one.