It'll restrict itself by having too many stems. The species form of the tree is slowly spreading, shrublike, and 'Nigra' is a more compact form. Apart from the occasional errant leader which might form, there should be no need to prune. If you really have to prune it, then you can now, and remove a few of the longest shoots. However, you will lose any flower on those, so you might want to wait until after flowering, and trim it to shape then. That'll give it all the growing season to make up the growth. Worthy

i would prune it out in early spring just before the buds swell myself x