It's much more correct to refer to the threat to disclose information in exchange for noncompliance with demands for an action as extortion rather than blackmail, and it's much more likely that the Israelis were extorting LBJ with the knowledge they had of his role in the events in Dallas just four years previously than the other way around.
Funny that with Jack Ruby/Rubenstein's self-professed patriotism for both Israel and the US, noone's much looked at the possibility of his connections to organizations other than the Mafia, particularly in light of his Chicago background. I wonder why that might be....
-archy-/-