Looks like a Cayuga, or maybe Cayuga mix? It looks like there is some brown on it's face, which would mean it is a mixed breed. Cayugas will be completely black with black bill and black legs. They do sometimes have some light yellow or gray on the chest like yours as ducklings, though.
My first guess would be Cayuga too. Could also be a Black Swedish or maybe a black runner. Do you know where the feed store gets the ducks? If you know the source you can see which breeds they have and maybe narrow it down.
Those were my two thoughts. She/he doesn't seem to stand as upright as a runner, though... She doesn't have much brown if any, I think its just the light, because she(I'm hoping its female, obviously lol) doesn't have much when I look at her, just in the picture.
Also, if she's single, she'll need someone for company 24/7. Don't panic. Some folks do raise singles successfully, but they understand that ducks are flock animals and go to great lengths to keep from stressing them if at all possible. Unbreakable mirrors help.