We know this:
- Trump is a bluffer who, when called on it, backs down.
- Trump said that Comey better think twice before talking about their meetings because, you know, "tapes" and all.
- Trump is mulling whether he will assert Executive Privilege in order to block Comey from testifying.
Now, there is the power and vengeance angle, which of course is not out of the question, but it would seem that with everything at his disposal, it would be pretty easy for Trump to discredit Comey without making a thinly veiled threat and never backing it up.
However, that same thinly veiled threat follows the exact same pattern as Susan Rice before she testified as well as the non-wiretapping by Obama. Bluster to try and redirect but then nothing when the surface is scratched.
If there really are "tapes" of conversations between Comey and Trump that would hurt Comey - or would help Trump, he could do any of a number of things. (Hint: one of them is definitely NOT to try and stop Comey from testifying). He could, you know, just release the tapes publicly. He could offer publicly to turn over transcripts, or pretty much anything other than never discuss them again while also letting it be known that they may stop Comey from talking.
This sends a number of messages, in addition to the obvious "why would he block Comey unless Comey had some very damning information" angle. First, it shows the very predictable pattern of behavior that will most certainly be taken advantage of by anyone who wants to manipulate this administration. It also shows that Trump plays a very strong but empty first hand in order to try and intimidate. It also shows that this is basically a full on bluff and anyone who waits this out or calls him on it will essentially have the upper hand immediately. Of course, this shows him to be an incredibly bad negotiator.
And in this case, it shows that there are no "tapes" of Comey saying anything that would either hurt Comey or help Trump here.