What if a prostitute . . . threatened to report a john as a form of blackmail?

They already can and sometimes do, usually as honeypots (here I mean the criminal kind). "Blackmail is illegal" and also blackmailing someone being very dangerous are two major elements preventing it. I don't think making buying legal would be a significant factor since usually the blackmail is on the level of social standing, not getting charged with a relatively minor crime (generally a misdemeanor). Furthermore, especially because prostitution exists more in the open in these societies, the prostitute who blackmails would also have her reputation damaged quite severely, to the point that it might not be viable for her to continue her profession if it gets out that she even attempted blackmail -- to say nothing of the fact that, not to beat a dead horse, having someone who absolutely hates your guts (the victim) makes being a prostitute much more dangerous: What if this is one of the old john's friends or someone he hired to hurt you?

"The plight of the johns" is also just not a very moving cause because anyone who is worried about getting blackmailed even given all of these factors can just not buy sex. Prostitutes are much more likely to be desperate -- though less likely in these countries than in a place like the US.