If it will emulate an SB28 on a film camera, there shouldn't be a problem. I did have a problem once with an SB28 resetting itself from ISO400 to 100, but it wasn't in TTL mode so this probably couldn't happen. However, was the ISO on the camera set correctly for the film (even with DX coding, I was always in the habit of checking it when I loaded film)? I also wonder about the negs - if they're properly exposed then it could just be a printing issue. There are two different TTL modes on the SB28 but both should be very accurate - even when shooting into a mirror (oops!).

F8, 1/60 and ISO400 should work pretty well for what you were doing. One last rambling thought - are they all evenly underexposed or not? If not, did you wait long enough for the flash to recycle? The SD8A battery pack makes this a no-brainer; the flash recycles instantly with settings like this.