Not so, PP. The term, Queen Mother, has been used unofficially to refer to the widowed consort of a king who is the mother of the current sovereign. You are right when you state that the widow of George VI incorporated that into her official title to avoid confusion with her daughter. Whilst alive, the Queen Mother was always referred to in royal circles as Queen Elizabeth, whilst Elizabeth II was simply The Queen.
Interestingly enough, before Queen Alexandra in 1910, you have to go back to 1669 to find the last Queen Mother, Queen Henrietta Marie of France.