My question is whether Block is right in saying that there is 'something wrong with the concept' merely because it can't be squeezed into determinism or indeterminism. An initial instinct may be that he is invoking the Law of the Excluded Middle, but isn't that inappropriate here?
This is obviously a contentious topic, there's no unanimous view, no consensus, on what free will is or if it exists. Thus, there's no definitive answer to your question, just different answers from different perspectives.
I'm a compatibilist, and in my view this Law Of Excluded Middle here, this "libertarian free will doesn't make sense in determinism, and also makes no sense in indeterminism", is one of the central motivations for why most compatibilists become compatibilists. I don't think it's inappropriately applied - if something isn't possible if A is the case, and that thing also isn't possible if not-A is the case, then... that's a perfect application of the Law of Excluded Middle here.
There's a reason most professional philosophers are compatibilists. I think this is central to that reason.