Falling Icicle
Adventurer
The dictionary isn't the only issue here. There are several decades worth of D&D tradition to think about. But even from the dictionary,
2. any knightly or heroic champion.
This doesn't seem that good a fit for a blackguard.
That dictionary definition was written for our world and history. One could reasonably expect that in a polytheistic world like most DnD settings, paladins would have very different origins than the monotheistic ones from Earth.