For a 256 KB save file, I imagine there would be no backup blocks. But the game was not meant to save files of this size, so I never bothered with them personally.
The Gen III save structure is sort of complicated... I am looking for the documentation I used to figure it out so I don't have to try to explain it (since I'm sure I'd only befuddle you if I tried). EDIT: Ah, look what a quick Google search yields: http://furlocks-forest.net/wiki/?page=Pokemon_GBA_Save_Format
The block footers are very similar between all Gen IV games... I think there are minor differences in HGSS, but nothing too complicated IIRC.
Your best bet for locating the PKM data in B/W is to just use a save file containing a few PKM you know the PID of, and opening the save file in HxD or some other hex program and doing a search for those PIDs.
Um, yes?
I don't think B/W switch the blocks around like Gen IV did, making it much more straightforward to find the "current" information. I dunno anything specific about the B/W save data though.
Also, don't forget that the Gen III PKM data looks different:
http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_data_structure_in_Generation_III
http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_data_substructures_in_Generation_III