Here is my pattern for a slouchy ‘boyfriend’ style cardigan for your Blythe doll.
I use one or two strands of very fine industrial yarn for this which is similar to a 2 ply or lace weight yarn. I knit loosely so I use a 2mm needle but if you are a tight knitter a slightly bigger size may be better.
You will only need small amounts of yarn for this, hard for me to determine how many yards!