A target has total cover if it is completely concealed by an obstacle.
And using the dictionary definitions...
concealed: kept secret; kept out of sight.
obstacle: a thing that blocks one's way or prevents or hinders progress.
While you're blanket does not hinder a sword from hitting you, it does hinder people from seeing what you are doing. Since you are "completely out of sight", You have total cover for purpose of line of sight. (Anyone who challenges this due to "Object permanence", just reference them to any stage magic. What you think you see is not the same as what you actually are seeing.)
Casting Time: 1 reaction, which you take when you see a creature
within 60 feet of you casting a spell
We already established that they cannot see you (even if they think they see you). They may be able to target "the moving mass under the blanket", but at this point, they cannot confirm that you are, in fact
A) Still within 60 ft
B) Are still a creature
C) Are casting a spell as apposed to rambling (debatable, but is something to think about in relation to bluff)
However!!! We should also consider another part of key wording
Casting Time: 1 reaction
It only takes 1 reaction time to cast the spell. So This could still be cast as a readied action, or simply as a reaction to what the mage believes they are seeing. So we really need to look at the valid target of this spell...
Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell
Range: 60 feet
Classes: Sorcerer, Warlock, Wizard
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a success, the creature’s spell fails and has no effect.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.
The word "attempt" means that this spell is a shot, not an affect. So you can cast it to no avail (IE, against nothing)... I don't see anything about line of sight, other then the implied one to get the reaction... it doesn't appear to actually require knowing where a creature is.
So what you really need is 2 blankets, a puppet, and learn ventriloquism so that they all blast your puppet instead of you. ^_^
Actually... forget the blanket. Jut learn ventriloquism. They can't target you because the puppet is casting, but they can't target the puppet because it is not a creature. :3