Which model of Blackstar are you using?

I put that 4CM guide together.  The reason I use -10dBV throughout the setup is because

if you use the 'line' (+4dBu) setting on the HDs FX Loop level switch:  you are sending a 'hot' input to the amp's input.  This whacks up the input level and can cause noise and unwanted distortion.

From what you describe:  it doesn't really sound like ground loop noise.  All true high gain amps hiss a bit to some degree or other, once you crank up the gain, but it should not be excessively loud.  On the ultra-high gain 'red' modes for my Marshall JVM I use a hard gate directly after the FX Loop block to trim off the high gain hiss.   When setup right it is very effective and not too intrusive..

I think your problem may relate to the levels set within the preset.   Manual troubleshooting may be needed.

Testing the Preamp gain levels

First off, unplug everything and turn the amp loop off.  Now plug a guitar into the front of your amp, select the clean channel and play it hard.  Adjust the gain so that the amp just starts to distort a little when you are plaing hard.Now turn the gain back to just below this level.

Now hook up the first two cables in the 4CM .  Setup for -10dB on the HD FX Level switch  Create a blank patch for troubleshooting, containing nothing but an FX Loop and the studio EQ that corrects for the loop level drop, setup as per my guide.   Again, the amp loop should stay off.

Play hard, again. Is it distorting?  Right, if it is then you have too much gain somewhere on the input stage and you need to back if off until you are clean again.  Suggestions:

• Use the input pad switch to cool off the input, if you have very hot pickups
• Try setting Inputs as Input 1 = Guitar, Input 2 = Variax.  This can cool off levers at the Pod's input.
• Mess with gain level on the Studio EQ

Testing the Power amp levels

Once you are happy that your Input is playing nice:  you can do something similar for your power amp.

Plug in the other two cables for your 4CM and set the 1/4" output switch on the HD to 'Amp'.  Turn the Pod master to full.

Stay on your clean channel.  Turn off the Amp loop initially. 

Turn on the Pod Looper and set it to work in the 'Pre' mode.  Record some hard strumming and set the loop to run.

Now try toggling the amp loop on and off.  The volume level should be the same in either position.  If it is not, adjust the Mixer level in the patch until it is (assumes the mixer is at the end of the chain.

OK., now with all of this done. Change to your high gain channel on the Blackstar.   Now in principle, your levels should all be balanced off and what you are hearing is probably as good as it gets.  if you still have lots of hiss then you may need to use the gate.  If you want to explore the ground loop possibility then try a Behringer HD400 or Ebtech hum eliminator in the loop.