From a legal standpoint blackmail requires the "receipt of money or valuable thing". Because the thing being received is an even exchange of goods already agree

From a legal standpoint blackmail requires the "receipt of money or valuable thing". Because the thing being received is an even exchange of goods already agreed to by both parties, and the threat on not receiving is not an illegal action in itself, it is not likely or plausibly blackmail. -not a lawyer, just work with too many of them