In this interview, David Crystal speaks with his usual good sense about blends. A blend is usually thought of as a portmanteau word, such as ‘motel‘ or ‘brunch‘. But in ‘The Cambridge Encyclopedia of the English Language’ he defines it as ‘The result of two elements fusing to form a new word or construction’ (my emphasis). The example he gives in the interview, and in his talk, is from a song by the Beatles, in which Paul McCartney sings of ‘the world in which we live in’. Those preparing his talk thought the repetition of ‘in’ was a mistake and tried to change it, but the point of his talk is that it isn’t a mistake. When we speak we do this sort of thing all the time. We start a sentence with one construction in mind, and finish it with another.
A good example which he himself uses in the interview is ‘the reason . . . is because . . .’ It’s enough to say ‘the reason . . . is that . . .’, but in mid-sentence speakers forget how they began it, and, because they’re aware that causation is somehow involved, they feel the need to use ‘because’ at some point. One of the best known pedants in the UK is the otherwise excellent broadcaster John Humphrys, but I have heard him, too, use ‘the reason . . . is because . . .’ (David Crystal’s long defence following the attack on him by John Humphrys sets the record straight on all sorts of misapprehensions.)
In general, as David Crystal says, blends have no place in the written language, but that is not to say they are incorrect in speech. The spoken language marches to the beat of a different drum.