Someone is misunderstanding the mechanic here and I'm not sure it's me.
http://www.wowhead.com/spell=180533/tainted-shadows Crushes a player with dark magic, inflicting 92500 to 102000 Shadow damage and triggering a Corrupted Blast.
http://www.wowhead.com/spell=180534/corrupted-blast Players struck by Tainted Shadows additionally inflict 83505 to 92296 Shadow damage to players within 5 yds.
http://www.wowhead.com/spell=180526/font-of-corruption Curse random players with dark power, causing subsequent casts of Tainted Shadows to additionally strike the Font of Corruption targets. Provides immunity to Corrupted Blast.
Yes, font of corruption players generate a corrupted blast but only because they're additionally struck by tainted shadows.
The initial tainted shadows target (1 player) also generates a corrupted blast. Now you could either have it hit that single player by spreading out 5 yards or stack up and have it hit every single person in the raid not font'd, including melee who'd normally never get hit. You're turning spot healing one person for 90,000 damage into aoe healing the half the raid for 90,000 damage.
Unless I've got it all wrong?