While it doesn’t solve your problem, the best workaround for now, I feel, is a single-select custom field but with one option value, not two. So your Blocked? custom field would be defaulted with no value and you have the opportunity to value it with a checkmark 
when true. Of course, in the API it’s still not really a Boolean but in the UI it actually operates nicely to avoid distracting until it’s turned on.
Larry