Not necessarily “dark”, but serious.
Carousel’s loser abusive husband, SOM’s NAZIs, King & I’s clash of culture.
Of course, often some of the best musicals were rather serious, but they usually were also hopeful or positive in some way. Not like those since 1965 which are putridly serious.
“You Have to be Carefully Taught” (racism) in South Pacific, and the degenerate Jud in Oklahoma.