A Dark Knight also known as a DRK typically uses a Great Sword, but there are swords that they can also use. I checked out the FFXI online Auction House and created a query - I tried to paste a link, but it doesn't work corectly. Go to ffxiah.com. On the left hand side you will see 3 search boxes and below them you will see in blue the words power search. Click on power search under the search box titled Item Search. Another page will open. On the right side there will be 3 boxes. Choose the job DRK, skip the race box and chose Main for the weapon slot. On the left hand side towards the bottom there is a search box titled Skill, click on the drop down and chose Sword. Then click submit at the bottom.
You will get a list of every sword in the database, but there is a little trick to pull in the swords that are over level 75. Click on the column heading that says LVL twice, that will sort the list by the level of the sword and also pull in the swords level 76-99......why, I have no idea - just a quirk of the database after the levels went above 75. You will be able to click on each sword and then there are link each file that takes you back to ffxiclopedia.com so you can get any further info you need. .