And I guess I always thought navy and black was a no-no because of seeing guys in navy slacks w/ jet-black suit coats and remember it looking a bit odd.
In Saxon style dress, it is a bit of a no-no to mix dark blue fabric with black fabric, because they're so close that they end up looking like you didn't have enough light in your closet when you got dressed! But still, that has no bearing on the black in your shoes or belt. Black leather is perfectly suitable with blue trousers (much better than brown, anyway!).

But you should toss Saxon rules out the window for Highland dress anyway... which I always find to be easier said than done. I actually kind of like the look of brown shoes with a lighter blue (like Lovat blue) hose. And black shoes go with anything.

If it were me in your shoes ...err, hose... I'd avoid black and dark blue hose, for the reasons auld argonian mentioned. To me, they start to look like the stereotypical old geezer at the nursing home who is wearing shorts with tall black dress socks.

And besides, the Highland tradition seems to be very well established toward middle-of-the-road color tones or even brightly colored hose. I could be wrong, but I can't think of any photos or portraits of Highlanders wearing dark hose until the last decade or two. Dark hose seem to me to be an attempt to incorporate Saxon dress socks into Highland style, and just throws everything out of proportion or off-balance. That said, I'm not a native Highlander or long-time kilt wearer or 'expert', and it's just my personal opinion. There's certainly nothing "wrong" with dark hose.