I think the blend values work differently from how you described it. It seems to control how much of the movement provider blends with a script (if a script is loaded). So it tries to combine/blend the randomness with the loaded script.
I noticed while playing a slow script and blend values set to 50%, the axis barely moved at all. When there is no movement on the R0 axis and there is movement on one of the other ones it tries to blend the random movement (which should be 0 because it’s based on the R0 axis) with the actual script resulting in very little movement.
It’s probably better to set it to 0% because you generally want the loaded script to not blend with the motion provider. This way when a script is loaded it just plays that script and if there is no script loaded it just does whatever the motion provider is set to.
If you want to limit the range of that axis you need to use the slider in the advanced settings instead.