Why did Christopher Robin call his father blue?

Could you elaborate on the reason behind Christopher Robin referring to his father as "blue"? Was there a particular incident or event that prompted this nickname? Alternatively, did it stem from a personality trait or a characteristic that his father possessed, which could be likened to the color blue? Additionally, was this a term of endearment or did it carry a deeper meaning within their family dynamics? Understanding the context and motivations behind this nickname would provide valuable insight into the nature of their relationship.