As someone mentioned, I would wonder if the fleece might stretch? Also, would think it'd block a fair amount of the light from coming in.
It'd sure do the job. I was raised in an old farm house, and Mom always had winter curtains that were heavier, then some light lacy ones for the summer.
Rather than fleece blankets, what about buying the yardage?
Not sure what length/height you need, but if you bought the fleece and turned it sideways, you could have one continuous piece.