More good slams are bid based on shape, fit, and shortness than HCP. Also, as a rule, Blackwood is a tool for staying *OUT* of bad slams, not getting to slam. This is a distinction that many don't grasp. Basically the idea being that you should be in grand with all four aces, and in six missing one, and in five only if missing *two*. If you are unwilling to commit to that, you are better of taking a more cooperative approach, using cuebids or the like. The voluntary raise to five is also an under-appreciated sequence.