Thay are flying away from the point of the blast at ~c, the speed of light, so all you'll be able to see is the stuff in the flat shell section you are in. You don't look back towards the blast point, you look out anywhere within the expanding shell. It looks flat and you'll only see part of the shell, because c always travels at the same speed and always looks like it does.
You'll never see Earth, because that's you. There are no mirrors out there. You'll just see the other lil-bits. One's that are far away look like they did when the blast happened, because the light from them took distance/c secs(years) to get to you and the time since the blast is approximately a little less than that.
If you look into empty space there's a light all around, coming from everywhere far away in the shell that is the red shifted glow from the initial blast intensity. THat's called the 3oKelvin(-273oC) background radiation from the blast. It's redshifted, because the speed of light must appear as a constant. To do that the frequency shifts to lower wavelengths. It's so far away, the time so long, that the light that used to be a very high frequency now looks super low, from an intensely cold source , almost absolute zero. It's like hearing a Harley engine as the rider flies off into the distance, the music from the engine goes to lower and lower octaves.
Since redshifts mean lower frequencies, time out there-far away looks like it is slowing to a near stop. Eventually all that stuff will hang out there in ever slowing motion, until it fades into the background radiation.