> I should not be required to go do hours of research to figure out
> how to fiddle settings so that it doesn't happen.

Agreed. You could submit the issue via report-emacs-bug.

The doc for frame-background-mode says "nil (automatic by default) if
you want Emacs to examine the brightness for you". It is not
evidently doing so in this case, because for me:

  emacs -nw -Q
  M-x customize-face RET minbuffer-prompt

shows medium blue rather than the expected cyan, using mate-terminal
configured to white on black in a Mint 12.04 install.

So Someone needs to investigate whether there's a way to determine the
terminal color settings and have the "automatic by default" behavior
cause the defined faces to use their "dark background" settings.