Jemandem das Blaue vom Himmel versprechen – to promise someone the blue of the heavens – is to promise the unreachable. It is an attempt to persuade via promises which have”weder Hand noch Fuss – neither hand nor foot.
To exaggerate, to paint a rosy picture of future developments is not in any way convincing to Germans. This might be one of the reasons why they are so sceptical about utopias of every kind.