Is the repentance of a murderer who secretly killed a Muslim then concealed his crime and repented accepted, or must he declare what he did to be punished for it?

There are three rights related to the intentional killing of a believer: the right of Allah, the right of the slain, and the right of the slain's guardians.

If the killer repents with sincere repentance, the right of Allah falls away.

If he surrenders himself to the guardians and they exact retribution from him, or take blood money, or pardon him, the right of the guardians falls away.

The right of the slain remains, and it is hoped that Allah will waive it for the killer and compensate the slain from His bounty if He knows the sincerity of the killer's repentance.

If the killer conceals his crime, neither the right of the guardians nor the right of the slain falls away.

It is a condition for repentance concerning the rights of others that the right be restored or absolution obtained from it. Therefore, the killer must inform the slain's guardians so they can decide whether to pardon, exact retribution, or take blood money.