Assymmetrical vs symmetrical voltage blocking
It is more accurately described as current blocking, not voltage blocking since voltage does not flow. Current does.
Asymmetrical current blocking means that it can only block current in one direction but in the other direction it can't block it and the current goes straight through. These devices can conduct symmetrically (in both directions). Kind of like a switch with a diode in parallel with it (like the parasitic anti-parallel body diode in most discrete MOSFETs).
So for something like MOSFETs to block AC you need two of them in series back-to-back so each one blocks current flowing in one direction.
Note that this is different than asymmetrical current conduction. This is when a device can only conduct it in one direction but might be able to block current symmetrically (in both directions). Kind of like a switch with a diode in series with it.
Thyristors are an example of this. For thyristors to conduct AC, you need to have two of them anti-parallel so that current passing in each direction goes through a different thyristor since current can't pass both ways through the same  thyristor.
An ideal switch or relay has symmetrical blocking and conduction. A diode has assymetrical conduction and blocking and such behaviour makes a third control terminal useless since there is no switching state that can be controlled independently.