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?
Out of curiosity, I looked to see if I could find any writing by Block on this subject as he is a fairly prominent professor. I found a freely available book he wrote "The Border Between Seeing and Thinking" and it does address free will and I think may help clarify.
Starting on page 459:
Before I end the chapter on consciousness, I will discuss one more topic: the relation between consciousness and free will. Although this topic is not usually discussed in the context of the nature of perception, I will be arguing that attention to perception clarifies a major controversy about conscious decision ...
At the end of page 460 it seems to get to the part you are asking about:
Some commentators conclude from this sort of evidence that the conscious decision to act is not causally efficacious in producing the action, because the unconscious neural events are sufficient to cause the action. This reasoning is my target. I will explain why this reasoning is mistaken in terms that apply to all mental events...
And a little farther on:
... examples that show that the conscious and unconscious aspects of a mental event can have different and opposed effects on behavior. When the behavior fits with the conscious aspect, we can sometimes be sure that it is causally efficacious even if the unconscious parts precede the conscious parts. The upshot will be that even in the cases where the influence of the conscious and unconscious aspects of the mental events point in the same direction, they may make somewhat independent contributions to the behavioral effect.
I don't pretend to speak for anyone else but as I read this, he seems to be arguing something along the lines that the whole concept of 'free will' is based on a false dichotomy. But my interpretation is uninteresting (and likely flawed) given you can read his text using the link above.