Hmm, it's a good example Leggings of the Black Blizzard is basically a "old world T5" level drop, if you consider MC/ZG at T1 = Kara/Mag/Gruul at T4.... BWL/AQ20 at T2 = SSC/TK at T5 ... and AQ40/Naxx at T3 = BT/Sunwell T6 ?
I believe the earliest upgrade / replacement is http://www.wowhead.com/?item=24359, give up some damage but the 2% hit is worth more than the crit, and the other stats definitely make it better. Following that there are some SMV/Netherstorm quest rewards and crafted stuff that's close or better, or BoE greens like http://www.wowhead.com/?item=24690 of the Invoker would be better?


Also I'll assume you mean the L60 Grand Marshal's Mageblade (http://www.wowhead.com/?item=23451) as the Spellblade is the blue L70 pvp item?
In that case a definite upgrade comes at L66 from Old Hillsbrad http://www.wowhead.com/?item=27431 or again some SMV/Netherstorm rewards.