Opponent has 5 colorless sources(one of them is a Cavern of Souls naming Eldrazi) and 1 Island. They cast Drowner of Hope and don't say anything, and I go to Disdainful Stroke it. He didn't announce he was using the uncounterable on the Souls, and because on the Island in play he could of used it just for colorless mana. Is it countered? When does he have to announce that?
Now if he has 5 colorless sources plus Cavern, I assume its uncounterable as the Cavern is his only blue source, but when he has another blue source plus Cavern, I don't know what mana source on Cavern he's using.
From Toby Elliot, L5, from 2012 (No more recent updates have occurred):
If a Cavern of Souls gets tapped for the appropriate creature type, the spell is uncounterable whether they say anything or not. We'd still prefer that players be clear about the game state, of course.
Now if he has 5 colorless sources plus Cavern, I assume its uncounterable as the Cavern is his only blue source, but when he has another blue source plus Cavern, I don't know what mana source on Cavern he's using.