If bitcoin is illegal, the only reason to engage in a black market transaction to obtain would be if it gave you access to material thing you could not otherwise get - unless there active hyper-inflation going on, no one would want to illegally obtain bitcoin just so they could turn around and buy some good they'd be able to obtain with normal, legal transaction.
All it takes is a decent return for savers since the INR is a terrible store of value. The massive gold black market in India is a clue that Bitcoin isnt going anywhere.
