Adding compositing effect to single object in EEVEE?

First time using compositor,
but how would i add blur node to just single (blue checker box) object in this scene using eevee?
Also a problem - losing its alpha when it needs to be mixed back in to the composite.

Edit note:
This is a simple recreation of my problem from another project, where i have whole animation done but i need to blur 1 object in couple of frames while its disappearing (its kinda like a VFX) and its in exact situation as this checker box, objects in front and objects in back.Checker box testblend.blend (710.8 KB)

Could you possibly render the object in “blurred” and in “non-blurred” form and then very precisely cut between these two strips of film? If, as you say, the object is “disappearing,” it might work. And this would give you the flexibility to precisely choose the “cut point,” if you render a few frames on either side of where you think it might need to be . . .