Note that he was charged with forgery too, giving some indication of what the blackmail content might have been.
Bahamas medic charged in Travolta extortion plot | Reuters: "Prosecutors allege the paramedic, Tarino Lightbourne, 47, tried to extort Travolta by means of threats. They also charged him with forgery."
Bahama Cops: Obie's Still in Our Crosshairs - TMZ.com: "As TMZ first reported, the subject of the alleged plot is NOT a photo. Rather, it is a document."