I think the fundamental difference is this: (in the most general terms)
Black power is a term used to describe a movement whose principal aim was to support and build up pride in being African American. While it certainly had a racist component (blame Whitey etc) it's purpose was not built around the subjugation and denigration of other races.
White Power, on the other hand, while containing many of the same elements (building and maintaining pride in a white culture, protecting an identity) has as its dominant theme the denigration and subjugation of other races as a principal purpose. This element is particularly visible in the White Power movement.
That being said, I think both groups contain disturbing 'jingoistic' and fascistic tendencies and elements that overpower them in the public eye.
At some point, as a species, we are really going to have to get past this 'race as identity', thing.
It really isn't that important. Culture, yes. race ... no. And they are not the same, although they are intertwined.