This question already has an answer here:
This post is a counterpart to this question: Is it known when and why Christians began to sign themselves with the Cross?.
Among the Christians who bless using the sign of the cross, is it known when and why imparting a blessing by tracing the cross over the ones being blessed began?
My thinking: Blessing and crossing oneself are two different things. Example: say in an audience with the Pope there are many journalists are attending of varied beliefs or of no belief at all. When the Pope imparts his blessing by tracing the sign of the cross, only those who cross themselves and who want to cross themselves will cross themselves. Meaning you can have one and not necessarily the other. My good guess perhaps it goes back to Christ himself or at least the Apostles.
And also there can be a blessing without tracing the sign of the cross.