I buy mine from Next or Burtons. They're black and I buy them in packs of seven pairs. Each one has embroidering on the upper part, so I can pair them. I have 21 pairs of socks in total. I hate mismatched socks but don't mind them so much if I'm just popping to the corner shop or parking the car in the garage. Anything else and they must match - hence the 21* pairs. Also means I can cycle through them every three weeks and gives me time to do laundry.
Ditto with boxers - 21 but as I don't need them in pairs, it doesn't matter.
I think I was told that they should match either the shoes or the trousers. Given that I only ever wear black shoes and black* trousers (in a variety of styles) black socks go fine.
+1. Completely frustrating.
*technically, there's fewer than 21 pairs as I'm missing a few random socks.
**work trousers are black. Suit trousers are different.