It's the decades-old problem of blue on black, which has led to interminable discussions of which exact tint of blue should be ECMA-45 blue on a terminal. Pick one, it has poor contrast with a black background. Pick another, it has poor contrast with a white background.
* https://invisible-island.net/xterm/xterm.faq.html#dont_like_...
So, pick two? One for each background?
There is only one 'blue', colour number 4, in ECMA-45.
A terminal emulator could choose to display different foreground blues, depending on the background colour.