If I understand correctly,
you mean that if you blur a layer, then apply an alpha track matte to it, the matte may do a sharp cutout instead of a blurred one ?
You can use an adjustment layer on top of the two layers, but it will of course blur every layer underneath it.
You can also apply a second blur to the trackmatte, but that adds to your rendertime, especially with gaussian blur.
IMHO Pre-composing would be the way to go.
Either precompose and then apply a blur to the comp, or precompose with the adjustment layer in the precomp