It was a 10 years-expired duplicating slide film.
And it wasn' t too dark, but the film was very slow, only 32 asa.
I set the shutter speed to 1/15 sec, which is the longest. After processing, I did not expect any visible photo when I saw the underexposed slides. After scanning them. I liked the results, since they came out in a deep blue. Totally worth it.