magic the gathering – How does “must be blocked if able” work with banding?

One creature can block both

Your assessment is that, because creatures don’t become ‘blocked’ until step 509.1h, creature B who is banding with creature A doesn’t figure out that it’s being blocked by a creature blocking A in time for the assessment of requirements during 509.1c.

But the same logic would imply that meeting any requirements in 509.1c is impossible, because at that point in the step no creatures have become blocked. Therefore step 509.1c must “look ahead” to what creatures will be blocked by the end of 509.1h.

Now, this might not be enough for one creature to block both two “must be blocked” creatures in a band if rule 702.22h (One creature in a band being blocked = all creatures being blocked) is a triggered ability. But it’s not. Triggered abilities always begin with “when”, “whenever”, or “at”, as per rule 603.1. Rule 702.22h begins with “if”. That makes it a replacement effect*, which means all banded creatures will become blocked at the same time as the original became blocked, during 509.1h. This means that 509.1c should be able to ‘look ahead’ to see it just like the original block.

*I’m not entirely confident on this point – the wording doesn’t quite match the way I would expect a replacement effect to be worded. But it’s certainly not a triggered ability, which is the main question here. If it’s not a replacement effect, it’s some sort of unnamed rules-internal effect that works similarly.