Can Trump block Comey from testifying?

He could. But, since the President already made statements about those conversations, it would be awfully awkward to claim executive privilege over the other side of the discussion. That would be akin to giving your side of the story, but not letting the other guy give his side - that would be bad form at least. If that occurs, I would expect and hope that the courts would be able to override executive privilege.

If he tries to keep Comey from speaking, that tells us who is telling the truth and who wants to hide the truth.

He can. But I don't think he will do it. Comey is his own worst enemy.