Ann: Falcon Intersects High Grade Gold at Ironbark East, page-43

The short answer is blade refusal is when ever the driller and geologist agree that they have an unweathered bedrock sample.

Blade refusal with AC drilling can be variable depending on the perseverance of the driller. The sample is returned up the centre of the drill stem as weathered rock fragments...a hole is terminated once the drilling gets to hard for the blade bit to proceed at a satisfactory rate. Most drillers will stop as soon as they get to recognizable bedrock (the geologists brief), but the name air core has a hiden meaning...if you continue drilling into competent bedrock, like an unfractured talc schist, you can return a sample resembling a diamond core. Geologists love this type of sample at the end of a cheap air core hole. Drillers offsiders dont love it, because the core breaks up and blocks the outlets at the top of the mast, meaning climbing up and disconnecting hoses, crearing blockages with a crow bar and compressed air, and chasing sections of core blown out the top and landing all over the paddock.

The consequences of getting it wrong can be significant. I have seen aircore holes terminate in hard compacted clays at 80m depth with practically zero mineralization, only to be drilled with RC later on into the same clays directly below and strike mineralization within a few metres.

This is rare detail you wont find every day and you should consider this when you judge the credibility of my next response.