Okay, some more info. The blood splattered in a 7-8 square radius around the waterfall, which matched the overflow from the drainage before the drains caught up.
The problem is mud - once the water receded, only the outermost tiles left behind blood, the rest just left mud. Compared to the original effect (ginormous amounts of blood on everything!), it's kinda sad.
I think what happens is that flowing water 'pushes' contaminants, but doesn't 'pull' contaminants. So the outermost tiles had no water flowing in from the outside when they drained and left behind blood, while the rest went down the drain. I'm attempting to set up a special room to test the theory.
#########
#G##G##G#
#########
#########
#G##G##G#
#########
#########
#G##G##G#
#########
Where # is floor and G is a floor grate (preferably made from a red stone, for effect).
Essentially, I'm going to make something like the 'mass pitting' article on the wiki, only instead of being used for dumping prisoners its for dumping water without disturbing the blood.
1. Get a reservoir of bloody water. Some weapon traps on a narrow bridge over a reservoir should guarantee a plentiful supply whenever the goblins come calling.
2. Drain the water into your "Altar To Armok" room - quickly, so that the room is evenly coated. It might be wise to make the floor of the reservoir a retractable bridge, so you can dump it all at once.
3. The water drops into the room. Each water tile is only 1 square away from a drain, so it immediately flows away without 'pushing' any blood out of the way.
If it works, the only 'blood free' tiles in the room will be the (red) flood grates. I'm pretty sure Dwarves track blood, so you might be able to get 100% saturation by pulling up the grates and replacing them with ordinary floor tiles.
As for weaponizing this, just make this room a 'secondary gatehouse' and use the nastiest forgotten-beast blood you can get. When a siege comes, close your main door and open your secondary door wiiiiiiide open. Anything barefoot will die. For extra nastiness, wait to dump the reservoir until they're in the room. Can't really take credit for this idea, since the "Reclaiming Battlefailed" thread did it first (and funnier), but what the hey.
On a somewhat related note, do elves wear shoes? I accidentally offered them a bunch of rock crafts in a wooden bin last season... Either way, I'll post again with screenshots if it works.