Does "30% less block chance" applies to spell block chance too?

The penalty applies to all base block chance, for attacks or spells.
"% of Block Chance applied to spells" is taking the block chance value (which has the penalty applied) and applying that as spells. The penalty is not applied a second time to this block chance.
But sources of base spell block, such as the linked quiver, will have it applied.

I've changed the stat description to make it more clear it applies to spell block.

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Semarogin wrote:

How does it interact with Necromantic Aegis? Will my minions get less block chance if I take Acrobatics?

No. You have acrobatics, not the minions. They have the shield's stats, not you.