Currently Blackguard has a unique class ability called 'Bull's Strength' which provides to them a +4 Strength bonus to both themselves and their summoned creature, what it doesn't suggest is that it -stacks- with the spell named Bull's Strength allowing you to get up to +9 strength between them, nowhere it this listed at all. Even worse is that since it has the exact same name as the spell, it'd be easy for a player to think that it was in fact, the same spell.
My suggestions is as follows
Rename The Backguard "Bull's Strength" to something more befitting the class (Profane Might/Unholy Strength/Something equally moustache twirling and evil)
Simply put, this should resolve all of the above issues with a very simple fix. The confusion between SLA Bull's Strength and the actual spell will no longer lead people to an incorrect conclusion in that they wouldn't stack. The tradeoff is that it'd be harder to use this ability openly, but given that it's got hours/level duration (From level 10) and using corrupt weapon similarly would out the character as being a blackguard, there's no real change or danger of the character being 'outed' from the ability in a way that they wouldn't already be trying to potentially avoid.