Hi edd,
well without any referenz noone will be able to help you, cause we canīt see what you did, but if I understand your your probleme right, you wanna make a object blurry itself and the rest of the scene uneffected by the blur right?
If yes, I think you rather wanna use Depth of field on the object.
Iīm not sure and still more of a newbie myself, but I think DOF (Depth of Field) will be best for what you wanna do.
But thats my oppinion and an oppinion without seeing what you are doing.