Are the culms (ie stems) green in the first year they emerge, going black gradually in the following year or two? In that case you probably have black bamboo (Phyllostachys nigra). If it now fills the pot, it probably needs dividing - cut it in half vertically, may need a saw to get through the roots, and put one half back into the pot with new compost. You can sell the other half for good money. You should divide it every second year.

If the culms are all dark pretty much as soon as they emerge, then you probably have Fargesia nitida. In that case it is flowering and will probably die and there is nothing much you can do to save it other than pray - don't feed it.

There was a garden open day in my local village this weekend, and both the Fargesia nitida specimens I saw were flowering. One was practically dead already, the other had only just started. This worldwide flowering episode started in 2003, and new plants grown from seed are already available.