Hi mate. I've seen some really nice minax arties on some players. Its good to know they can spawn there. Out of interest did you have a luck suit or anything? I farmed Blackthorns quite a bit when it first came out but I've not been back for a long time now. I got 200 points, collected some items and that was my lot!
As for if its worth keeping I would trade it in myself. The 20 DCI is nice. Maybe if you have a fencer a bit short on cold resist and MR who is wearing a luck suit then it might be the jewel for you!
As far as I'm aware (which is my personal experience, obviously), Luck didn't affect the properties on the Minax drops at all. Granted, I wasn't really looking to use them, I just wanted the 100 turn-ins for a Hooded Robe of my own (was borrowing a guildmate's Luck Conjurer's.) I did, however, notice that, even while hidden, I got more Minax drops while wearing my Luck Suit (1870). In one day of farming Fel Blackthorn's Mages, I pulled 15 Minax drops.
While it's entirely possible that the RNG just decided to be VERY nice, that wasn't the first time I farmed for Minax items - when I was working on my sampire's suit, I went down there for the Mace & Shield Glasses clone, getting the Norse Helm when I got my 50th. At the time, I didn't wear any Luck, and I had to wait longer between drops.