It is generally regarded as ineffective against a rational opponent who has or is an ally of someone who has assured destruction capability. In this situation if the opponent refuses to respond, then one's choices are either surrender or suicide. Hence during the Cold War, the explicit threat of nuclear warfare to force an opponent to perform an action was rare in that most nations were allies of either the Soviet Union or the United States.
The United States issued several nuclear threats against the People's Republic of China in the 1950s to force the evacuation of outlying islands and the cessation of attacks against Quemoy and Matsu. The unwillingness of the Soviet Union to respond to these threats was one of the major factors in the Chinese decision to develop an independent nuclear arsenal.
Interestingly, nuclear blackmail is considered most effective when the person making the threat is not rational and is willing to commit suicide. (See game theory). The prevention of these threats by irrational actors is the stated purpose behind the National Missile Defense undertaken by George W. Bush in the United States.