Perhaps I am missing something, this seems to be an accurate model of opportunism of which blackmail itself is a subset. Namely, both examples given are a type of opportunism which the agent exploits the target's desire for a particular value (either reputation or a MacGuffin) for profit, despite the risks of lost value balanced against the target's anticipated value. Blackmail, colloquially, is typically used to denote the above by use of incriminating information (such as steamy letters), while purchasing the MacGuffin may be called any number of things depending on the character of the act.