It's not an error. Blue backs are rather well known on 1937 and 1954 $1s but they didn't start out that way. My understanding is that putting them through the laundry (in a pocket, say) with a particular detergent caused this colour change.
Green is the result of mixing blue and yellow pigments; take out the yellow chemically, and voila! a blue back.
The composition of the green ink used on the faces must have been different because the green seems to survive the reaction that changes the back colour.