Blue-Sky Thinking, written "blue-sky thinking" in the dictionaries, is defined in Cambridge Dictionary's Business English section, under the adjective blue-sky, as "ways of thinking about a subject or dealing with a problem that are new and original but may not always be practical."
Dictionary.com's British entry, from Collins English Dictionary, lists "blue-sky thinking" as a noun meaning "creative ideas that are not limited by current thinking or beliefs."
Oxford Learner's Dictionaries defines the adjective blue-sky, marked "[only before noun]," as "involving new and interesting ideas for things which are not yet possible or practical," with the example "The government has been doing some blue-sky thinking on how to improve school standards."