negative charisma does not effect the duration of fear in any way.
while on the subject, i dont see why people would think it would. to me a highly charismatic necro would be able to scare something better than one who wasnt charismatic "um... boo... please run"
A pair of these earrings nets you +6 STR, +6 INT. The same result can be achieved (though undoubtedly at greater expense) without the charisma hit by wearing one gold sapphire and one electrum sapphire earring. However, I will grant you; it's quite characterful for a Cazic-worshipping, unrepentant, mean old shadowknight to wear these, and it probably costs a lot less to make provided you can find a shaman to imbue the stones. If you're interested in more than just the raw numbers, and want something to distinguish your character, here you go.
If the earrings granted AC, ANY AC, they would definitely be the item of choice though. Some of the imbued jewelry is somewhat underpowered (that is, doesn't provide a significant extra benefit for the trouble it takes to make imbued jewelry) when compared to existing jewelry.
Another positive side of this for us "unrepentant Shadow Knights" is that we LIKE neg CHA as it improves the effectiveness of fear. If I weren't already wearing cougar claw earrings I'd have a set made for me.