Hope this pic helps. From this you can see a female guppy about one or two days away from birthing. Notice the dark gravid spot that you have been talking about, rather large and 'squarish'.
I read your other thread, and if you are planning on using a box/net breeder then either get a net one, or be sure to change the water in the plastic box every day because in my experience the boxes can restrict the flow and sometimes a film will build up on the surface of the water in the box/net so agitate that too. don't fully submerse them in the tub thing you had because feeding will be a problem and also lack of surface contact may or may not be a problem.