It is legal to keep
D. couperi in New York, though the state requires you obtain a permit first, as they require such for all ESA listed species, even those which are non-native. Also, should you purchase a specimen outside your state of residence, you will be required to obtain a USF&WS interstate commerce permit as well. However, I am aware of at least one individual breeding
D. couperi in New York, so you may be able to acquire one locally and forego the USF&WS interstate commerce permit, but yuo would still be required to obtain a valid state level permit.
Here are links to two sites which will provide more information to this regard:
www.indigosnakes.com
www.indigosforever.org
Best regards,
Jeff