Blackswell is a hold name. This mechanic will exist for all 5 colors, so there'd be Redswell, Blueswell, Whiteswell, and Greenswell.

This probably has rules issues. Also, it asks players to float mana around as if everyone knew exactly when the mana pools are emptied.

Yeah, unfortunately you can't respond to the trigger if written this way. Though you could write it this way:

"Whenever another creature you control dies, if you have Blackswell, each opponent..."

The problem is that you have to target with the way it's written, and I'm pretty sure that target will fail if you don't have Blackswell. If you don't target, though, the ability will only check for Blackswell on resolution.

I should point out, these details will be lost on many players. Even if the ability does work, some people will assume it doesn't unless you Blackswell before the creature dies. Thankfully, creatures rarely die for no reason, and enough people will get the idea that they can blackswell in response to a spell or ability. A few won't know that.

Another thing that many people don't understand is how and when mana stays in your pool. This creature is going to confuse a lot of players when they Blackswell in combat, trigger the ability, then empty their pool at the end of combat.

I will say, for what it's worth, I find the ability interesting, and could be fun. Wizards development team would probably shoot it down. But if you're cool with the ability not being realistic in the real world, it might be the sort of ability that makes custom sets worth playing.

Also, missing p/t (assuming you weren't going to add that in later.)

Actually you can respond to the trigger written this way - it's the intervening if-clause that stops the ability to trigger in the first place.

The problem that players will not know which is true is the real issue though - also the floating of mana at inappropriate times leading to bad gameplay AND the fact that a mechanic that only works once you have ten mana is also not going to play that well.

Note: Why is blackswell a keyword? Shouldn't the reminder text belong to the triggered ability? Compare devotion.

Edit: Nah forget it. I rescind this comment because I'm probably wrong.