I ordered another roll of natural filament from Amazon.com and tried that.  It hit the same issue virtually immediately and when I retracted it, the end was covered in blue filament.

After fighting with it for several days (and I mean many many hours!), I finally decided to take off the extruder nozzle.  Despite being very careful, some of the ceramic broke off and the temperature monitor wire popped off.  I carefully cleaned everything thoroughly until I could drop a pin down through the filament tube and have it come out of the other end clean.  There was plenty of blue filament in there and some caked on right at the bottom of the nozzle.

I reassembled everything and fed the blue filament into the printer again.  Guess what?  It got back into the same state of being unable to pull the filament through.  Since I had already spent most of my afternoon on it (again), I was about to give up completely.  I decided to clean everything one more time but this time use the other roll of natural filament.

It was only after I switched back to the natural filament that the printer was able to pull through the filament.  I am thankfully now back to being able to print.  I will not be using anything but the natural filament from now on.  I'd rather spend a little extra time painting my creations rather than risk getting into the situation I've been in for the last week.  I don't think the extruder could survive another cleaning attempt - the ceramic paste is cracked and falling off and the temperature monitor wire is barely staying where I reattached it.  The acrylic plates are all ugly now too from all of the brushing and scraping I had to do to get all of the blue filament off.