As a wedding gift (many years ago) we received a lovely soft blanket that was made with fibers imbedded into a sheet of foam. After a a few years and many washings the fibers were all gone and I was left with a sheet of foam. I used it as the batting for a picnic blanket and it was perfect. My grandmother also used old blankets as batting in her quilts all the time. As mentioned before, this was during the depression when people made use of everything they had.
So, yes, blankets will work but you'd want to take the precautions mentioned above so they lay flat and don't have worn/thin spots.