Blocker trigger

[Lorn Asbord Schutta]

How to properly word trigger that counts each instance of a creature your opponent controls and is not blocking the creature with ability? I guess if the blockers are declared simultaneously it is easy, something like:

Whenever ~ becomes blocked, for each creature defending player controls that isn't blocking ~, [effect].

However, if the blocking happens in sequence (I block with this, and I block with that, and...) then the above wording gives more instances of [effect] then I want. And I have no idea how to phrase it differently.
So I guess the question boils down to the way blockers are declared on stack.

[Mookie]

There are two separate things to consider here. One is the 'when ~ becomes blocked by a creature, [effect]', which will trigger once per blocking creature - see Kessig Forgemaster // Flameheart Werewolf. The other effect is 'when ~ becomes blocked, [effect]', which will only trigger once - see Ib Halfheart, Goblin Tactician. It sounds like you want the latter.

For example: "whenever ~ becomes blocked, it deals 3 damage to each creature the defending player controls that isn't blocking it."

[Lorn Asbord Schutta]

There are two separate things to consider here. One is the 'when ~ becomes blocked by a creature, [effect]', which will trigger once per blocking creature - see Kessig Forgemaster // Flameheart Werewolf. The other effect is 'when ~ becomes blocked, [effect]', which will only trigger once - see Ib Halfheart, Goblin Tactician. It sounds like you want the latter.

For example: "whenever ~ becomes blocked, it deals 3 damage to each creature the defending player controls that isn't blocking it."

Right, but should I make something like:

"whenever ~ becomes blocked, put a charge counter on it for each creature the defending player controls that isn't blocking it."

or a p/t change, then it would trigger for each creature blocking it, or not? And if former, would it see the final number of nonblockers, or (all creatures defending player controls) - (those, whose block was already resolved)?

Nevermind, it is getting late here and I start to think too slow. Thank you.

[Mookie]

Right, but should I make something like:

"whenever ~ becomes blocked, put a charge counter on it for each creature the defending player controls that isn't blocking it."

or a p/t change, then it would trigger for each creature blocking it, or not? And if former, would it see the final number of nonblockers, or (all creatures defending player controls) - (those, whose block was already resolved)?

The relevant rule here is 509.5 - there are multiple subrules for different types of triggers.

509.5. Triggered abilities that trigger on blockers being declared may have different trigger conditions.
...
509.5c An ability that reads "Whenever [a creature] becomes blocked, . . ." generally triggers only once each combat for that creature, even if it's blocked by multiple creatures. It will trigger if that creature becomes blocked by at least one creature declared as a blocker. It will also trigger if that creature becomes blocked by an effect or by a creature that's put onto the battlefield as a blocker, but only if the attacking creature was an unblocked creature at that time. (See rule 509.1h.)

509.5d An ability that reads "Whenever [a creature] becomes blocked by a creature, . . ." triggers once for each creature that blocks the named creature. It triggers if a creature is declared as a blocker for the attacking creature. It will also trigger if an effect causes a creature to block the attacking creature, but only if it wasn't already blocking that attacking creature at that time. In addition, it will trigger if a creature is put onto the battlefield blocking that creature. It won't trigger if the creature becomes blocked by an effect rather than a creature.

So whether it triggers once or multiple times depends on the exact wording of the trigger.