color: Do not use dark blue in dark-background palette

color: Do not use dark blue in dark-background palette

In GNOME Terminal's default dark colour schemes, the default (dark)
blue on a black background is barely readable.  Light blue is
significantly more readable to me, and is also easily readable on a
white background.

In Konsole, rxvt, and xterm, I can see little if any difference
between dark and light blue in the default dark colour schemes.

So replace dark blue with light blue in the dark-background palette.

Signed-off-by: Ben Hutchings <benh@debian.org>