Phantom blocker blocks Phantom power while leaving the audio signal intact. Phantom power can be applied to either the male or the female side of Phantom Blocker's xlr connector and will not reach the opposing xlr connector, the audio signal travels without restriction in both directions.
There are microphones (mainly older ribbon types) which do not tolerate Phantom power. These microphones have a center tap on the step up transformer which causes a current to flow through the secondary windings of the transformer. This current magnetizes the transformer core which has a negative effect on audio performance.
Connecting a ribbon microphone which can normally tolerate Phantom power to a cable which has Phantom power applied sends a rush of current through the microphones transformer, this current gets amplified by the transformer into the primary circuit of the microphone and forces the ribbon out of the magnetic field it is suspended in, this can permanently stretch the ribbon.
Some (older) preamps are sensitive for Phantom power at the input. transformer cannot handle Phantom power from another source. The transformer and his very thin silk isolations can burn out because a burst of Phantom power switching on creates a high voltage in the input transformer ,a complete blocking is needed.