Nothing about this sounds abnormal other than the sight glass looking relatively clean throughout the backwash process.
I wonder if it's just not picking up enough of the flow to show what's in hte pipes - does your backwash line go into an underground pipe? Mine is only 3 feet long and empties onto a large flagstone before it spills onto the lawn and the water that comes out upon backwashing is relatively filthy (grey) looking and compares to what I see in the sightglass 1 for 1.