"blurry" can mean many different things, do you have an example of a song that does this
#Song effect
halcyon garden
this is a low pass filter, with the Cutoff / Frequency being automated
go to your mixer and pick the insert you want this effect to happen to (i'm going to just assume you want the Master insert)
add a filter plugin, fl has a few, the easiest to use is Fruity Free Filter, since it's already set to a low pass
right click the Freq knob and create automation clip
draw the automation clip how you want the control to move