See: HERE and HERE for examples.
The White House's official position is that of 'Sgt. Shultz', "I know nothing!". See HERE; and here is obama's personal quote on the matter: “I’m not familiar with this notion that anybody has been blocked from testifying. So what I’ll do is I will find out what exactly you are referring to.”
The "secret" is in the wording used by both obama and Carney: "blocked". Threatening someone is NOT "blocking". Threatening is "If you do xxx, then this is what will happen to you!", which is what has been alleged. "Blocking" is taking 'em off the playing field, as it were. I call these types of answers from this administration "lies of obfuscation".
One has to be very careful on what is actually being said.