Interestingly, the ruling on this has gone back and forth a few times, and the archived SCG ruling is the one I went with. Chris is correct (i.e. Falter does not prevent Spiders from blocking), since he pretty much always is, but it’s SO counter-intuitive. I had to think this rules issue through, when I stumbled upon another card that made me realize why Falter works this way. The most recent “Falter” effect (Incite Hysteria) had its Oracle text changed before it was even printed.
From:
Radiance ? Creatures that share a color with target creature can?t block this turn.
To:
Radiance ? Until end of turn, target creature and each other creature that shares a color with it gain “This creature can?t block.”
The reasoning behind this is because the original version of the spell works like Falter by creating a static ability that lasts for the entire turn. If a creature changes color (or enters play) before becoming declared as a blocker (via Quickchange or the upcoming Aethermage’s Touch, perhaps) the static ability will still check to match its colors to the original radiance colors to determine whether the creature can block. The Oracle text grants the ability to the creatures themselves, making it a one-shot ability that only affects what’s currently in play (as it currently exists).
Falter’s text is like the original wording of Incite Hysteria. It affects every creature on the board until end of turn, regardless of that creature’s properties or whether it came into play before or after Falter was cast. Hence, when a Spider goes to block after being Faltered, the game treats it “as though” it had flying, and thus allows it to block. I have a feeling that if Falter were ever reprinted, it would read, “Until end of turn, creatures without flying gain, ‘This creature can’t block.’”
If I’d remembered Incite Hysteria, I would have realized sooner how Falter currently works.
