If you were sold those as Black Bamboo (Phyllostachys nigra), I think you were conned. I'm pretty sure they are Fargesia nitida, which doesn't have a common name. I think you will agree that the stems of what you bought were actually a dull dark grey rather than the shiny black of real Black Bamboo. Also young culms on Black bamboo are green not black: they turn black in the second year, and I don't think you have any green culms. The acid test is whether the culm sections have a groove in them - real black bamboo has a vertical groove ("sulcus") in each culm section on alternating sides of the culm, and F nitida doesn't.

Disreputable sellers sometimes sell F nitida as black bamboo, because it is black(ish) and a lot cheaper. Though F nitida is a very nice plant in its own right. The plants you have are dying because they have flowered. Anyone with any specialist knowledge of bamboo knows that Fargesia nitida is in flowering mode, and it is necessary to sell seed-raised post-flowering stock to avoid it dying from flowering.

Take them back to the seller and demand your money back. If that is not an option, then they do sometimes revive, though as a much smaller plant, that will probably take about 5 years to get back to the size of the plant you bought. Or maybe really what happens is the seeds fall off and a few germinate and give the impression of the plant reviving, having a similar timescale. Water them as necessary, but don't feed them heavily until they are growing well.

If, however, I have misinterpreted the picture and you do have black bamboo, then come back to us and we'll try to work out what killed it. You shouldn't need to water them over the winter if they are planted in the ground.