[tompazourek]

They are not demanding ransom. Ransom is (per Merriam Webster): "a consideration paid or demanded for the release of someone or something from captivity".

They copied the data, and they want money otherwise they will release it. It's ordinary blackmail.

[bigbubba]

Perhaps you could say they are ransoming the exclusive ownership of the data. But yes, 'blackmail' seems like a better fit.