Actually... again, as I said; that only applies to mana abilities or abilities that need to actually be 'cast' by tapping or other such means.

Blood Seeker's ability is a 'triggered' ability which is completely different from the norm.

"603.2a Because they aren't cast or activated, triggered abilities can trigger even when it isn't legal to cast spells and activate abilities. Effects that prevent abilities from being activated don't affect them."

But it's just a casual game, so it's not that big of a deal.