Actually, I screwed up slightly. The Penny Black was first issued on 1 May 1840 and was valid for postage from 6 May; the twopenny blue was issued on 8 May and was valid immediately. The twopenny blue is a much rarer stamp, but the penny black has that cachet of being the first (though only by a few days) and has a certain symbolic importance too, since everyone refers to the early prepaid postal service as the "penny post". (Though the standard penny rate only applied up to half an ounce. Today's standard rate allows up to 60 grams, a shade over two ounces.) The penny black also didn't last very long, as it was replaced by the penny red the following year.