Is it permissible to intentionally make an illegal move in order to bluff?
No, you are not allowed to do this because doing so would be a violation of tournament rules at every level.
At Competitive and Professional REL, the Infraction Procedure Guide determines how judges handle rules violations. The action described in the question is a Game Rule Violation, a sort of catch-all category that includes making invalid attack or block declarations. In addition, this action is dangerously close to Cheating as defined there:

A person breaks a rule defined by the tournament documents, lies to a Tournament Official, or notices an offense committed in their (or a teammate’s) match and does not call attention to it.
Additionally, the offense must meet the following criteria for it to be considered Cheating:

The player must be attempting to gain advantage from their action.
The player must be aware that they are doing something illegal.


The action described involves knowingly breaking a game rule for the purpose of gaining an advantage.

At Regular REL, the Judging at Regular REL document reigns. In that document, the &quot;Serious Problems&quot; section lists the following:

Knowingly breaking or letting an opponent break game or tournament rules, or lying, in order to gain an advantage. “Bluffing” about cards opponents can’t normally see is permitted.


If Bob instead tries to declare the block and then takes it back all within a single action, it's no longer reasonable to interpret it as an illegal action. Instead, he's just thinking out loud through the applicable requirements and restrictions that determine blocking legality. So the rest of the wouldn't apply and it should be fine for Bob to do that.