Yet another situation where negative speech may be in order is where one is the victim of false accusation and wishes to divulge the identity of the real culprit to vindicate himself. Whether or not this is permissible depends on the nature of the crime.
If the crime is of a type that would halachically require the observer to inform either the victim or others of the culprit’s identity, he should do so.
If there is no constructive purpose in revealing the culprit’s identity other than to vindicate oneself, it would be forbidden for the accused to name the culprit. The accused should declare his innocence and refrain from incriminating anyone else.
In a situation where only two people are possible suspects, which means that a denial on the part of one is tantamount to an accusation against the other, it is permissible to deny the charges provided that the act committed was indeed improper. However, if the alleged offense was, in fact, an innocent statement or action, the accused should not deny it. Denial would implicate the other person who, in this case, is guilty of no real crime.
It is a middas chassidus (measure of piety) to accept the blame in all cases, unless revealing the identity of the guilty party seves a constructive purpose. Also, one should not accept the blame for an act which would reflect badly on the community or on observant Jews in general, as this would constitute a chilul Hashem, desecration of God’s name.
Sefer Chofetz Chaim, 17 Kislev, Page 202