blue-sky

BLUE-SKY (adjective} 不切實際的
- concerned with thinking of new and interesting ideas, without worrying about whether they are practical or not :
blue-sky thinking, blue-sky research, blue-sky theory

Example:
The Education Bureau in Hong Kong has been doing some blue-sky thinking on how to improve school standards.

Believe this 2 two word phrase "Blue-sky" is open for interpretation in many aspects. When you say it's a blue sky holiday it could literally means : Bright and sunny; refreshing ; non rainy or dampening holiday.

While a blue sky thinking could means: openess, unclouded and clarity in thinking.

(lol) I've google'd it but find no exact definition.