that the DBI has much, much more material on Trump
.we've seen only the tip f the ice cube. And it is possible that Whitaker could see it. And there is the actual danger -- that Whitaker will, to the best of his inability, try to recite that information while reporting back to Trump. And that is exactly the level of corruption that those in the DoJ/FBI who are actively investigating Trump know was a primary goal in Trump's appointing him.
Yet this is distinct from the concept that Whitaker could blackmail Trump. Again, having access to viewing the information in no way provides him hard copies of it. Thus, at very best, he could threaten to say he saw evidence of Trump's corruption. But that is, by its very nature, an empty threat. Perhaps the only people who would be listening to anything Whitaker could say (without backing evidence) would be federal investigators and the FBI. If Whitaker is ever charged, he would lawyer-up. No attorney would okay attempting blackmail -- the only possible exception being Rudy, who already represents Trump. Thus, Whitaker would have to try to do so on his own, knowing that the federal investigators will be monitoring everything he does. Paul Manafort could assure Matt of that reality. Thus, such an attempt could and would ONLY result in more federal charges, and insure a long incarceration.
Trump would have no reason to fear such an extremely unlikely threat. He would know that investigators already have the information, which would allow him to quickly refer any threat to blackmail him to the DoJ. The scenario you are suggesting has no basis in reality.