This is the context where I read this:
Quand les arbitres de L1 ont un petit coup de blues, ils peuvent jeter un œil à ce qui se passe au Pérou
what's the meaning of "coup de blues" here ?
Un coup de blues = A small amount of blues.
It's a phrase meaning a little bit down. Essentially it's "feeling a little bit blue", with the same use as in English.
That idiom means to feel slightly depressed and is linked to the English "to have the blues" from the African-American music.
Have you checked the Wiktionary ?
There is a dedicated entry for "Coup de blues".
And it means :
Moment pendant lequel on éprouve de la mélancolie, une tristesse passagère un peu floue.
Be careful the plurial is :
Coups de blues
It is having a down day or like missing someone deeply