In a ship like the Cutty Sark, eyebolts come in many sizes. Generally speaking, the bigger the rope (or wire) that's to be run through it, the bigger the eyebolt. A good average size for the iron rod that forms the eyebolts might be in the neighborhood of an inch, which translates into .010" or .012" on 1/96 scale. For larger eyes on which there will be more stress (e.g., the ones to which the lower ends of the main and main topmast stays are secured), a thicker wire would be in order - say .010".
A cotter pin would be the right shape, all right, but I don't recall having seen any that were small enough. In any case, as we've established, it's ridiculously easy to make your own eyebolts - at virtually no expense.
Several chemicals for blackening brass and copper are on the market. I've got a bottle of "Blacken-It," which I got in the railroad department of the local hobby shop. Bluejacket (www.bluejacketinc.com) sells "Brass Black" (along with "Pewter Black" and "Brass Brown"). Some modelers have gotten excellent results with gun bluing, which can be found virtually anywhere rifles are sold - including Wal-Mart. In any case, I suggest spraying the finished, blackened part with a quick shot of Dullcoat (or other flat lacquer). The black finish tends to rub and/or scrape off otherwise.
Hope that helps a little. Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.