I would say yes, Black is still worth getting. I have done the same thing with Black/White and Black/White 2; I played the BW2 before I ever touched BW. Despite this, when I finally played the original BW, I found the games different enough that it was worth getting.
*Some locations do not exist in BW (for example, the underwater tunnel used for travel in BW2 or the sewers under Castelia).
*The BW Unova Pokedex does not have three-hundred Pokemon from the beginning. You also cannot capture Pokemon from previous generations at the very start, forcing you to use the newer ones.
*There are not new forms of the three roaming Pokemon from Black and White: Tornadus, Thundurus and Landorus.
*BW takes place two years before BW2 and the role of Team Plasma changes a little.
*All the gyms have changed in some way, as have some of the other key battles:
The first gym depends on which starter you choose, as opposed to BW2's first gym being Cheren and his Normal type fiasco.
The second BW gym is Normal type run by Lenora instead of the second gym being Poison and run by Roxie.
The third, forth, fifth, and sixth gyms have the same leader and types, but the layouts are completely different.
The seventh gym is now Ice and is run by Brycen, as opposed to being Dragon and run by Drayden.
The eighth gym is Dragon, instead, and the leader depends on the game version (Black's eighth gym leader is Drayden while White's is Iris; they still possess the same Pokemon).
In BW, Alder is the Unova Region Champion, not Iris. The Pokemon used by Alder are mixed, providing a little more of a challenge than an all-Dragon team.
You still battle Ghetsis, but his Pokemon have changed entirely.
You do not battle Colress, but instead, you fight N (his team varies a bit, depending on the game).
*Keldeo does not have an alternate form in BW.
The starter Pokemon are the same, but the hero and rival(s) are different (the whole thing with your rival changes, being a bit different from all the previous Pokemon* games).
*Your fabulous journey starts in a town called Nuvema Town.
*The 'Free Space' to sort the items from your bag in BW2 does not exist in BW.
*Items cannot be sorted as they can in BW2.
*Using an Everstone while breeding does not pass down the Nature to the offspring as it did in BW2.
*The sacred White Forest has wild Pokemon, now (I guess this doesn't matter for Black/Black 2, though, huh?).
*Pokemon Dream World has less Pokemon than it.
I think those are all the differences. Please tell me if I missed any!
(Source, Source, Source, Source, Source)