blast in proof: blow up while being tried. Soft! i.e., wait a minute, let me think. The king is worried about the fact that they have to get their plot right the first time they try it, without a chance to test it beforehand. Ironically, the plot does suffer a "blast in proof"; almost everything goes wrong. Hamlet does die, but the guilt of both Laertes and the king is revealed, and they die too.