Not necessarily mislabelled. All of the culms of P. nigra shoot green, and stay green in the year they shoot, at least. Then they mature to black, usually in the summer of following year, if they are sufficiently exposed to the sun. Though this may fail to happen if they don't have enough sunshine, and also very young plants are more reluctuant to turn black. If you only have 1m high culms, this is a very young plant, so give it a while yet. Feed and water it well for rapid progress, especially in the late summer and early autumn, as this is when it stores up resources for the following year's shooting.

There are varieties of P. nigra that aren't black, and sometimes people buy these in error thinking that they are black bamboo. This is not widely understood, I've even heard experts on gardening programmes who didn't realise this. If your nigra is described as "Henonis" or "Boryana", it isn't black at all - the former is dull green, the latter green with brown spots.

Unfortunately, because P nigra sells at a premium (and expensive to propagate), the unscrupulous do sometimes try to defraud people by misdescribing something cheaper (and easier to propagate). The most common scam is to sell Fargesia nitida, whose culms turn grey after shooting green. (A very nice plant, but not P nigra.) You can easily distinguish between Phyllostachys and Fargesia, as the anatomy is quite different. Phyllostachys has two branches at each node where branches appear (though sometimes one branch immediately splits, giving the appearance of three), and a groove on the culm between each pair of nodes, called the sulcus, alternating from one side of the culm to the other. Fargesia has no sulcus, and a large number of branches at each culm node. Google will find you some reference pictures no doubt. If you have a juvenile Phyllostachys, very hard to prove whether it is nigra or something else - an expert might be able to give a positive identification by very close examination of the shooting culm.