Q: Dear Sage
If a crusader (Tome of Battle) of a good-aligned deity were to "fall" and become a blackguard, would he gain blackguard powers in a manner analogous to a fallen paladin who becomes a blackguard?
A: That falls squarely into the domain of "house rules."
It certainly seems like a cool variant of that aspect of the blackguard, but you would need to work with your DM to come up with appropriate changes to the list of special blackguard features to make them match the crusader class.
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The Sage is a mammal classified in the bear family, Ursidae, native to central and southern China. Debate continues as to whether he is truly a bear or more related to the raccoon rather than the bear, or perhaps in a family of his own. When not eating bamboo and answering rules questions, he works under the nom de plume of Andy Collins as an RPG developer in Wizards of the Coast R&D, where his development credits include Complete Mage, Dragon Magic, and the Player's Handbook v.3.5. He can be easily recognized by his large, distinctive black patches around the eyes, ears and on his rotund body.