A very common way of saying you are suprised is saying you were "shocked", but thats usually more on the negative side. Another good one is "astonished".

Blown away - positive. Taken aback - negative. Stunned - you can use it for positive or negative.

More or less. "I was taken aback" is neither formal nor informal. It is literally is a reference to ships and sailing. (Many people don't know this). To be "taken aback" means the wind suddenly changes direction, the sails are blown against the masts, and all forward motion is stopped. So, in normal use, it means "I was so surprised that it stopped me completely." You didn't react at all for a moment. It is negative in the sense that you were expecting to move forward and were forced to stop. It's not negative in the sense of being something terrible. "The description said that the campsite had electricity. I was taken aback when I found that my plug wouldn't fit the socket. I had to make a trip to a hardware store for an adapter." Other ways to say it are: "It stopped me in my tracks." "It stopped me dead in my tracks." "It stopped me cold." "I was stunned" literally means you were hit on the head so hard it made you dizzy. This, too, has the idea that you couldn't think for a moment, you didn't know how to react. It can be positive or negative. "I never expected to get into that school and I'd actually forgotten I applied. I was stunned when I got the acceptance letter." "I was blown away" is informal. The literal reference means "destroyed by an explosion." Oddly enough, yes, it is usually positive.

"I was blown away" means that I was very impressed by something, such as an artistic performance. So, yes, it is positive. "I was taken aback" is more more like mild shock: you hear or see something so unexpected that you don't know how to respond. This could be either, but it is often negative. You might be taken aback by somebody's rude answer to a question. "I was stunned" could be either positive or negative, while 'It was stunning' is nearly always positive. Want another phrase? How about 'gobsmacked' (speechless with surprise) or 'gutted' (surprised, disappointed, devastated). Both are common, informal phrases in BrE. You're bound to come across them in interviews and reality shows featuring ordinary folks.

Not always but almost always. I can say "I was blown away by how poorly she performed" but I would normally say "I was blown away by how great she performed". "I was taken aback by her great results." That is OK but normally it is negative, "I was taken back by her poor grammar" for example.