"I have a few -- ah -- items at home that might embarrass me, if the Ministry were to call...as you can see, certain of these poisons --"
     --  Lucious Malfoy to Mr. Borgin

Dark Magic items are magical artifacts that contain sinister, dangerous magical powers. Some, like the Hand of Glory, simply give light, but are obviously created using evil magic; the Hand actually uses a real human hand, for example. Other Dark Magic items are cursed, like the opal necklace in Borgin and Burkes. 

The Diary of Tom Riddle

My sister used to commit her innermost thoughts to her diary. Her great fear was that someone would read it. That's how the idea came to me of a diary that is itself against you. You would be confiding everything to pages that aren't inanimate. 
     -- J.K. Rowling (interview in Sydney Morning Herald, 10/2001)

One of the more interesting and dangerous Dark Magic items is the diary of Tom Riddle. This simple diary, originally purchased by Riddle in a Muggle shop on Vauxhall Street in London, was bewitched. Tom Riddle, who would soon thereafter become Lord Voldemort, placed some of his own thoughts and intentions into the diary, hoping to preserve the work he'd done for years to find and open the Chamber of Secrets. The diary appeared blank, but when someone wrote onto it's blank pages, Tom Riddle as he was during his years at Hogwarts wrote messages back. Lucius Malfoy had kept the diary for years until he had a chance to plant it on Ginny Weasley, slipping it into her Transfiguration textbook. Tom used the diary to influence Ginny into opening the Chamber again and loosing the Basilisk. Harry Potter defeated this memory version of Tom Riddle by plunging the broken, poisoned fang of the Basilisk into the diary. When he did this, all the ink spewed out of the it and with it poured out the Dark Magical enchantment..

Borgin & Burkes of  Knockturn Alley sells quite a variety of Dark Arts magic items (CS4). They include:

• a withered hand on a cushion called "The Hand of Glory" which, when a candle is inserted into it, gives light only to the holder

NOTE: The Hand of Glory is drawn from English folklore, described as a withered human hand whose fingers, when lit like candles, cast a charmed sleep on members of the household so the thief can rob the house. [WEB LINK]
• blood-stained playing cards
• a staring glass eye
• evil-looking masks
• human bones
• rusty, spiked instruments
• long coil of hangman's rope
• opal necklace which is cursed and has claimed the lives of nineteen Muggles

• shrunken heads
• live gigantic black spiders
• poisonous candles
• human fingernails
• Flesh-Eating Slug Repellent