As I understand, the reason down is such an effective insulator is that it traps small pockets of air and it is difficult for warmer or cooler air to circulate through because of that. As a result, if your down blanket were not contained in an airtight textile it would not provide much insulation. And if you did have an airtight textile, probably it wouldn’t matter if it was filled with down or anything else, the insulating effect would be more or less the same.