There are situations where you need to have multiple randomized animations played one after another, for instance, when your character is standing in place and performing some random actions. To be able to set the parameter responsible for randomizing the animations, you may need to transition through a neutral Idle pose. In that state, you can set the parameter value safely. See the following image:

Additionally, if your random actions blend together nicely (maybe they are different versions of the Idle animation), you can allow the parameter to change when you are already in the Blend Tree. To do so, make sure to use the SetFloat() function with the dampTime parameter set to an appropriate value (0.5 should be okay, but you will need to experiment).