The plural form of the word blue

“The blue” is sometimes used in literary speech or writing to mean “the sky” or “the sea”. Does its plural form indicate both the sky and sea?

For example, is it a meaningful sentence if I say “the horizon is where the blues intersect”?

Nothing's wrong with it. It even has a kind of literary value. Making it clearer is always advisable when your listeners don't get it, but if they do, then just go ahead. It sounds a bit off just because it is not used as frequently as it is in literature, not because it is inappropriate. It is not lexically incorrect, neither is it grammatically flawed.

“Blue” or “the “blue” can take a normal plural when used as the name of a colour, e.g.

“The blues and greens in the painting are especially evocative.”

It has various figurative usages such those mentioned (much rarer alone in English than in languages such as French) where the plural is never used.

It is more common in an idiomatic sense meaning “nowhere” — esp. “Out of the blue” (perhaps an allusion to the sky”) but again is never used in the plural.

One obvious reason why “the blues” might sound strange is that it is an idiomatic expression for “depression”. However the figurative use of “the blue” to mean “the sea” refers to it in a general sense (as opposed to the land) rather than to a specific sea, such as the Mediterranean. Hence there is no plural.

Using it to mean both the sky and the sea in one sentence is not English usage and to me sounds initially incomprehensible and then absurd.