Unrealistic and impractical: "[The author] shows what is testable physics, what is philosphy's domain, and what is blue-sky nonsense” ( Ann Finkbeiner).
Having a cloudless sky; clear: a blue-sky day.
To hold or express unrealistic or impractical views, especially in estimating something. Often used with it.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.