I probably shouldn’t trust the sign so implicitly because grammatically, it’s incorrect. It should read: “The world’s first stamp, the Penny Black, was sent from Bath.” But we’ll let this go.
As stated in the above sign, the very first stamp was the Penny Black—so called because (A.) It cost a penny, and (B.) it was black. Why get all creative and stuff with product names when you can keep it plain and simple?