From the discussion on Paladin codes:

A Fallen Paladin can choose to trade-in their levels for equivalent Blackguard powers.

Does this apply here?

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You must always apply for Blackguard levels.

All applications are considered on a case by case basis. That is, we may or may not allow this to proceed, depending on numerous factors.

It may be worth considering that a blackguard will always have other class levels also, since it's a PrC, so the transition, if it were permitted, would not be tidy, class wise.

The chances of a paladin turning into a blackguard in EfU is incredibly remote.