I was watching the television show Gunpowder, Treason & Plot, a series about Mary Queen of Scots. On episode shows her giving in to her lover Bothwell and allowing him to kill her husband. Later, she is in the confessional and confessing to this sin and the priest tells her that murder is a mortal sin and cannot be forgiven. Was this ever so? I have been given to believe that all sin may be absolved so long as the sinner is truly repentant.
No, there is not now and was never any such thing as an unpardonable sin.
However prior to the 1917 Code of Canon Law, Church laws could be a bit scattered and overlapping. There may have been local rules that reserved forgiveness of certain sins (e.g. murder) to the local Bishop or a specific cleric but it was still forgiven through that confessor. Considering that Mary’s confessor testified in her defense against the charge of murder, my guess is that this confession scene was just something that was done for dramatic effect in story telling and is not based on historical fact.