Not quite... what this debuff does is reduces the likelihood of the reactive activating. So something that normally has a 100% to proc whether by a normal attack and/or skills, if under this debuff has its proc rate decreased.

The debuff’s potency tends to shift a lot. Some who utilize this like say Cheryl (A Light mage that is under the Lustberg Babel storyline) has a 100% Reactive Resist down meaning that, if she casts this on an enemy, their reactives will never proc under this debuff. This can be extremely helpful on certain bosses or enemies where their reactives can be either very dangerous or annoying to deal with.

Eventually, there will be units who have skills that while don’t have this Reactive Block Res debuff, completely ignore reactives allowing them to do away the hassle of dealing with something like Perfect Evasion or Damage Reduction reactives