Can an Obj-C Block execute itself?

Question

This is an extension of this queston: Is it possible to create a category of the "Block" object in Objective-C.

Basically while it seems possible to create a category on blocks, either through NSObject or NSBlock, I'm having trouble understanding how the block would be able to evaluate itself. The example given in the answer to the last question:

- (void) doFoo {
  //do something awesome with self, a block
  //however, you can't do "self()".  
  //You'll have to cast it to a block-type variable and use that
}

Implies that it is possible to somehow cast self to a block variable, but how would one execute the block itself? For example, say I did a category on NSBlock and in a method did:

NSBlock* selfAsBlock = (NSBlock*)self;

Is there any message I can send to selfAsBlock to have the block evaluate?