Football Association official: like the majority of fans, we are also eager to punish those black whistle who accept bribes. But cracking down on black whistle depends on real evidence, not speculation, otherwise there may be unjust, false and wrong cases. 
Referees suspected of being black whistle usually show some injustice on the football field.
 some people guess that some referees are black whistle or shouldn't be based on some phenomena without evidence.