Black Bamboo is a 'clumber' not a 'runner' so it will always grow closely packed together. The advertisements all say Black Bamboo stems will get 2 inches in diameter and 25 to 35 ft. tall. I have never seen a Black Bamboo that big outside of Japan. In the California Central Valley, the stems got 1 inch in diameter and about 10 feet tall. The entire plant was about 4 feet by 4 feet and that took 20 years from a 1 gallon pot.
In San Francisco at my father in law's house, it was pencil thickness and about 3 feet tall. Here in Reno, the stems are about 1/2 inch in diameter and the plant is about 6 feet tall.
If you want large stems that are spread apart, you will probably have to look for a 'running' bamboo. Maybe even a timber bamboo. Don't expect any bamboo to grow to its full potential in your area.
This is my Black Bamboo plant, its been in the ground for 3 years now and the base is about 2 feet across (the original plant was about 2/3's that size). As you can see, it is not un-clumping, just slowly getting wider. Emphasis on slowly.