Yes, subs up port back is the best way to make the box. Or subs up port to the side.
1st off according to your sig you have (2) PA 3000's amps and you also plan to add (2) large Kinetic 2400s. That takes up alot of room right there. You will have to make a amp/battery rack on the passenger side of the box with amps slightly angled stacked on to top of each other and the batteries under the amps.
If the box is 30" long that's putting it about 2" away from hitting the rear hatch, well for me anyways it is.
If you make the box 30" long you will have to make the box with subs up and port firing to the side.
You should try to find the exact measurements of the cargo area for a 4dr blazer. That way I can compare it to my 2dr blazer.