What camera are you using? A Lomo of some sort? If that's the case, they have a set aperture and shutter speed, right?
Also, what do you mean by completely black? Are you looking at a photo CD, the actual negatives, or minilab prints?
Many factors going on here... If you're talking about scans/prints, then perhaps you took these pictures inside? If that's the case, ISO100 film + the "slow" lenses typical of a Lomo type camera would definitely result in black images. Take a look at the negatives, are they more or less clear? (save for the orange of the film obviously)