There is no definitive answer as to whether or not a blackjack weapon is illegal in New York. It is ultimately up to the individual municipality to make any determinations about what constitutes a weapon, and as such, it would be difficult to say definitively whether or not a blackjack weapon is illegal in New York.
That being said, it would generally be considered inappropriate and potentially dangerous to carry a blackjack weapon in public, so it would likely be considered illegal if someone were to do so.
