my testing has revealed the same usage regardless of what staff your blocking with. There is no 30% reduction I can see. It does however drain magic so that works.
1) the cp reduction
2) jewelry enchants
3) magica drain
4) the amount you can block (from ancient knowledge) appears to be working. the amount blocked is lower with a frost staff over a lightening staff showing the extra mitigation is working.
5) dk passive
I also tested sword and board and everything appears fine there.
just the 30% cost reduction not kicking in when you block with the ice staff and have "ancient knowledge" (2).