more vim stuff - invisible blue on blue in 7.0

[BizarroJack]

I have a problem when I'm using syntax highlighting with the default color scheme, and using vimdiff. I am using vim in a terminal window. For a line where comments exist in one version but not the other, The text is blue because that's the color of Comments, and the background is blue because it's a line that's in one file but not the other. In vim 6.x, the :highlight rules look the same for "DiffAdd" and "Comment," but somehow it decided to make the text white when both rules applied. In vim 7.0, it follows the instructions more like you might expect, and just makes fg and bg both blue.

:highlight shows the same thing on every system I use, but only my gentoo system with vim 7.0 has blue-on-blue text.

DiffAdd xxx term=bold ctermbg=4
Comment xxx term=bold ctermfg=4

Anyone know why this is? I suppose I can just change the color, but for some reason I find that philosophically unsatisfying.
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Jack