K'rrik's ability doesn't modify or reduce costs you pay. It changes only how you may pay those costs.

If a cost contains a mana symbol that may be paid in multiple ways, such as {B/R}, {B/P}, or {2/B}, you choose how you'll pay it before you do so. If you choose to pay {B} this way, K'rrik's ability allows you to pay life rather than pay that mana.

You can't pay 2 life to pay for generic mana in costs you pay, even if an effect says that you must spend black mana to pay that generic mana.

K'rrik's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

K'rrik's ability lets you pay 2 life for {B} in any cost you pay, including the mana costs of spells, activation costs, and even costs for special actions (such as morph). Any time you pay mana, that's a cost.

A Phyrexian mana symbol contributes 1 toward the mana value of a card, even if life is paid for it. Specifically, K'rrik's mana value is always 7.