I propose that Contagion be given the Inflict Wounds treatment - applying it on the Blackguard's weapon (or ammunition) grants a on-hit property that applies some sort of malus.
Per the second part of the suggestion, Blackguard summons should be able to interact with infected targets.
By attacking an infected target, they could receive something like one of the following:
- receive a large burst of healing while subsequently removing the malus
- receive smaller heals on-hit: see weapon vampirism when attacking the infected targets - this method would leave the malus on the target
- AC and CON boon + a regeneration effect upon the summon killing the infected target