Detraction

In Roman Catholic theology, detraction is the sin of revealing another person's real faults to a third person without a valid reason, thereby lessening the reputation of that person.

Distinction from calumny
Detraction differs from the sin of calumny and the civil wrong of defamation, which generally involve false accusations rather than unflattering truths.

The Catholic Encyclopedia clarifies:

Detraction is the unjust damaging of another's good name by the revelation of some fault or crime of which that other is really guilty or at any rate is seriously believed to be guilty by the defamer.
An important difference between detraction and calumny is at once apparent. The calumniator says what he knows to be false, whilst the detractor narrates what he at least honestly thinks is true.