A high quality down blanket encloses your own body heat well and it is also breathable. Real down shields you from the temperature of your sleeping environment. 

Many people mistakenly think that a thick and heavy comforter equals a warmer comforter. But this is not necessarily the case, because it depends on the type and quality of down. Duck down is heavier and has a lower insulation capacity compared to goose down. Goose down is a lighter type of down with a higher insulation capacity. This means that more duck down is needed in a comforter compared to the same down blanket filled with goose down. The duck down comforter will therefore be much heavier and thicker than a goose down comforter. But therefore not warmer. 

Be well informed and don’t let yourself be fooled by a smooth seller.