No you cant. Block/parry doesnt work while you are casting. Never did. The same way the runemaster rr5 was useless if you mocced or tried to nuke while a tank on you, you didnt evade.
Thats why the opponent wins a slam duel most of the time when you try to nuke him while he is running to you and you WANT to get the last cast get off.
Yes you can block while casting, but as someone pointed out, as soon as you block, you are interrupted for your next cast (delay on your shield). If you are primary tank in your group, it really locks you down to being a block bot and your cast damage is nullified or reduced greatly.