Reuters has an excellent article that gets into this question.
WHAT DO WE KNOW ABOUT THE MEN IN BLACK?
Witnesses and grainy video footage revealed armed men with assault rifles and M-79 grenade launchers appeared under cover of darkness on April 10 during a heated standoff between red shirts and troops trying to break up a protest in Bangkok's old quarter.
The government says the rebels, who wore black and covered their faces with hoods and balaclavas, appeared in the crowds of protesters and opened fire on troops, triggering chaos and prompting panicked soldiers to fire back in self-defence.
Government officials and the army believe the men in black are politically aligned with the red shirt movement and sought to cause bloodshed severe enough to force Prime Minister Abhisit Vejjajiva to call a new election.
Red shirt leaders say the "black clads" are protecting them, but they don't know who they are.
They're protecting us...but we don't know who they are. This strikes me as being very much of the times, a sort of inverted version of the Iraq war's icon, the fake policeman.
The Reuters piece also digs into the back story some distance. A possible explanation is that the men in black - surely a better translation - are former members of an army unit created in the 1970s to fight insurgents, as part of the broader cold war counter-insurgency strategy. The force in question is the Rangers - the name is important, as US advisors created the same force in several countries, apparently unaware that the term's cultural associations aren't particularly resonant outside the US. How do you even translate it?
They were eventually disbanded 10 years ago, which suggests that there may well be people plying for hire or else just on the political market generally who served in them. Interestingly, quintessential modern thinker Thaksin Shinawatra was politically close to the army officer responsible for their creation.
Also, meet the community-radio hackers of Thaksinite rebellion.