=3 I'm in a web-designing class... A slightly transparent background is great, If you have a non-interesting picture underneath; that picture is VERY INTERESTING, therefore you need to define the area around the text by a box; but the box needs to be very transparent to define both the area around the text and let you see the picture. I also recommend a thick border around the bar-code. =3
P.S. the Text over the box doesn't need to be transparent at all. If you give a little bit of transparency to the viewer, they want it all. The best thing to do is make it "Pop" by leaving it fully nontranslucent.