No, he couldn't have been.
He was born in 1936 and Black Sabbath was most famous in his 40's (1970's).
In the picture he is wearing a cassock, which clearly does not normally have Black Sabbath images printed on top, and he's clearly in his late 20's or early 30's.
In particular, the poster depicted as the t-shirt image
!t-shirt image
shows the Master of Reality logo from 1971. The pope would have been 35 in that year.
!enter image description here
However the picture without the poster is from 1966, way before Black Sabbath was even founded.
> An image from 1966 of the Argentine seminarian Jorge Mario Bergoglio in the El Salvador School, where he taught literature and psychology in Buenos Aires.