A blanket is a large square or rectangular piece of thick cloth, especially one which you put on a bed to keep you warm.

A blanket of something such as snow is a continuous layer of it which hides what is below or beyond it.

You can refer to something such as an unpleasant emotion or an undesirable quality that seems to affect every aspect of a particular situation as a blanket of that emotion or quality.

If something such as snow blankets an area, it covers it.

You use blanket to describe something when you want to emphasize that it affects or refers to every person or thing in a group, without any exceptions.